NEW YORK -- When Paul Avignon and Bob Butts decided to open a carryout below their Sumptuary restaurant at 400 Third Ave., they considered -- and rejected -- a lot of menu ideas. Originally, they wanted a meat-and-potatoes kind of place, but they decided the concept would be too heavy, a turn-off for the upwardly mobile (and weight-conscious) clientele they hoped to attract. Then they thought about serving just meat loaf and mashed potatoes. Too limiting, they concluded.

"Why not just mashed potatoes?" Butts mused.

Thus was born Paul and Bob's Mashed Potatoes, a place where spuds star and toppings as diverse as shiitake mushrooms, curried vegetables and seafood provenc al are used to fashion a meal out of one of the most comforting of comfort foods. Here, the potato theme spills over onto the dessert list, which features (what else?) baked sweet potatoes, and a suave and exotic purple yam ice cream.

The limited menu concept has less to do with anyone's nostalgia for mashed potatoes than the fact the restaurateurs wanted to do something completely different in the fast-food carryout field. "Everyone was doing baked potatoes," recalls Avignon, who adds that he "ate more pasta than potatoes" growing up anyway.

That Paul and Bob's potatoes are presented as entrees and not merely accompaniments isn't all that distinguishes the team's raison d'e~tre from the competition; the kitchen prepares the appropriately lumpy potatoes with bits of skin, which Avignon claims make his dish taste "so potatoey." And while few would equate mashed potatoes with dieting, the partners claim to have lost a combined total of 55 pounds since opening the restaurant in April. Avignon's secret? Mashed potatoes made with skim milk (a menu staple) instead of butter and cream, and seasoned with a dash of white pepper for esthetics.

Paul and Bob's amazingly homey potatoes are easy to make: Simply wash the spuds, cut them into large chunks (leaving the skin on) and immerse in cold, salted water. Bring to a boil and continue to cook until the potatoes can be easily pierced with a fork. Drain, remove to a mixer and beat in butter, salt, pepper (preferably white) and cream or half-and-half to taste. Diet mashed potatoes are cooked the same way, but include only skim milk and pepper to taste.

From Paul and Bob's Mashed Potatoes comes the following recipe for Mushroom Topping. Button mushrooms can be substituted for pricier shiitakes, if desired.

Express Lane list: mushrooms (preferably shiitakes), olive oil, garlic, white pepper, parsley MUSHROOM TOPPING FOR MASHED POTATOES (4 servings as a topping)

1 pound mushrooms (preferably shiitakes), sliced lengthwise

3 tablespoons olive oil

1 tablespoon butter

2 to 3 cloves garlic, pressed

Salt and (white) pepper to taste

1 tablespoon chopped fresh parsley

In a skillet, saute' the mushrooms over medium heat in oil and butter for approximately 10 minutes, or until mushrooms have released all their juices. Add garlic and cook a minute longer. Season to taste with salt and white pepper. Sprinkle with parsley and serve over mashed potatoes