HEAT oven to 350°F. COMBINE egg whites, water and cream of tartar in large mixer bowl. BEAT with whisk attachment on high speed until stiff but not dry, just until whites no longer slip when bowl is tilted.
BEAT egg yolks and salt in small mixer bowl on high speed until thick and lemon-colored. Gently but thoroughly FOLD yolks into whites.
HEAT butter in 10-inch nonstick omelet pan or skillet with ovenproof handle over medium-high heat until hot. POUR IN egg mixture; gently smooth surface. Reduce heat to medium. COOK until omelet is puffed and lightly browned on bottom, about 5 minutes. (Lift omelet at edge with spatula to judge color.)
PLACE pan in 350°F oven. BAKE until knife inserted halfway between center and outer edge comes out clean, 10 to 12 minutes. LOOSEN omelet edge with spatula. SERVE immediately.
This simple French-inspired oven-baked soufflé omelet can be made savory or sweet. Fill it with cheese, meat and vegetables or top it with fresh fruit compote and whipped cream.Puffy omelets begin to deflate when removed from oven, so plan to serve immediately.To serve folded: Using sharp knife, cut across center of omelet, cutting through upper surface but not through to the bottom of omelet. Top with filling, if desired. Tip pan. Fold omelet in half with turner and invert onto warmed plate with a quick flip of the wrist. Cut in half or into quarters.To serve open-faced: Invert pan over warmed plate, or slide omelet from pan onto plate. Top with filling, if desired. Cut in half or into quarters.