ONCE UPON a time, spinach was the vegetable most disliked by most people and -- especially -- most children. When I was growing up in London, I agreed completely with the famous cartoon in Punch, the satiric weekly, showing a mother furiously upbraiding her young son at the table: "I say it's good for you and you're going to eat it!" The small boy, chin out, replied, "I say it's spinach and I hate it." Then there was an American agricultural expert who said, "If God had intended us to eat spinach. He would have flavored it with something."

Spinach took its first big step toward popularity -- at least with me, as a boy -- with the cartoons of Popeye the sailor, who "fights to a finach, 'cause I eats me spinach . . ." Popeye would produce a can of spinach, drink it down as if it were root beer, the muscle in his right arm would swell to Herculean proportions, and with one blow of his fist he would stop an express train at full speed or flatten an army of villains.

But spinach, many years earlier, had been accepted by the rarefied arbiters of French high cuisine. That country's most famous gastronomic philospher, Brillat-Savarin, was mayor of Belley at the edge of the Jura Mountains. Every Sunday after attending mass he would lunch with the priest, Canon Chevrier. This holy man, a brilliant cook, had discovered a way of simmering chopped spinach in butter for five days so that as water was expelled from each pound of spinach, it would absorb at least 3/4 pound of butter. The final result was a luxurious, velvety-smooth dish of spinach.

More recently, spinach was given its crowning glory in French high cuisine terms by the late chef Fernand Point of the three-star Restaurant de la Pyramide. He lightened buttered spinach by working whipped cream into it, and eliminating the slight bitterness of the spinach by adding a few pinches of sugar.

But today, in terms of the strictly low-calorie "cuisine minceur," such things as butter, cream and sugar are dirty words. So it is proper that the ultimate modernization of an elegant spinach dish should come from chef Michel Guerard, the inventor of cuisine minceur. In his recipe, he replaced butter, cream and sugar with -- of all things -- a puree of Bartlett pears! To understand the brilliance of this anticaloric idea, you have to know a little about this pear.

It was discovered in 1770, growing as a wild tree in England, by a school-teacher on a Sunday stroll. He told a friend named Williams, a nurseryman, who took cuttings from the wild tree, propagated them, and soon was selling Williams pear. That name still is used throughout Europe. The tens of thousands of bottles of pear brandy and pearl liqueur made in France, Germany, Italy, Switzerland and Spain all are called Williams pear.

Guerard estimates that each portion of his spinach dish has only 150 calories. I decided to publish it not for that reason but because it is an imaginative way of serving spinach, with an unusual and memorable balance of tastes. MICHEL GUERARD'S PUREE OF SPINACH AND PEARS (4 servings) 2 tablespoons fresh lemon juice 2 pounds ripe Bartlett pears or, less good, other soft eating pears 2 pounds young spinach leaves, tough stalks discarded, or (less good) equivalent frozen. Coarse crystal sea salt or kosher salt and freshly ground black pepper

About 1800, some pear trees were planted on an estate near Boston. The next owner, Enoch Bartlett, did not know the name of the pears growing in his new garden. So he sold some young trees under his own name and that was how it became the Bartlett pear in the United States. It is at its best, and at its lowest cost, during August, September and October.

Put about 2 quarts cold water into saucepan, and stir in lemon juice. Peel and core pears and quater lengthwise. Drop into lemon-water and heat to a gentle simmer. Simmer until pears are just cooked, usually about 2 minutes. Drain and reserve.

The perfect cooking of spinach goes so fast that it works best with two persons -- one washing it and the other stirring it with a long wooden spoon in a big, deep pot on the stove.

Plact pot over high heat. Wash handfuls of spinach under fast-running cold water to remove all sand. After washing, do not shake out the leaves, just throw them into the pot. The only coating liquid is the water o the leaves. Keep adding handfuls of spinach to pot as fast as possible. As each handful falls into the pot, press it down hard with the wooden spoon, at the same time sprinkling in plenty of salt. Within several minutes, all the spinach will have wilted. Remove pot from heat at once. Let spinach cool in the pot until it can be handled.

Using hands, remove spinach from pot and squeeze out as much juice as possible. Place spinach on chopping block and coarsely cross-chop it.

Put pears and spinach into food processor. Process until mixture is smoothly pureed, but not liquified, usually in about 6 to 9 seconds. Taste puree, working in a good grinding of black pepper. Gently reheat it. At this point, if there is a bit of liquid, simmer the puree gently a few minutes to cook off excess water, or drain it.