Forget what you concluded about hotel restaurants being boring; the Mayflower has hired an extremely talented young chef and given him free rein to show what he can invent in the New American mode. So the first course might be an extravagant and impeccable seafood tart with not one but three sauces, or a sweetly fresh corn soup. There are seafood terrines of charming hues and flavor, and even more inventive soups. Main courses are garnished with baby vegetables and wonderful sweet potato pur,ee, and include the likes of outstanding veal with sweetbreads and kidneys, or liver -- unfortunately a little bitter but perfectly cooked on my last visit -- grilled to a slight smokiness and sauced with raspberries. Seafoods are cleverly mated and beautifully cooked. And desserts look gorgeous, yet don't disappoint in the taste. Here is an American kitchen that is using the new vegetables -- multihued peppers, fiddleheads -- in lovely ways. The dining room is elegant, with silver service plates and napkin rings, and plenty of handsome flowers. The flaw in this garden of Eden? Service that loses you in the shuffle and has on occasion been downright rude. Hh prices and high art are teamed at Nicholas, but clearly the food is the thing that makes this new endeavor worthwhile. 1127 Connecticut Ave. NW. 347- 8900. L $14-$18.50, D $17- $24.50. L daily ex Sat, Sun, D daily. AE, CB, DC, MC, V. Res sugg. Full bar.