4 large eggs, separated
1 tsp. unsalted butter, melted, plus 1 tsp. unsalted butter
1/4 tsp. salt
1/4 tsp. cream of tartar
¾ C. Filling
l oz. Parmesan cheese, grated (1/2 C.)

Adjust oven rack to middle position and heat oven to 375 degrees. Whisk egg yolks, melted butter, and salt together in bowl. Place egg whites in bowl of stand mixer and sprinkle cream of tartar over surface. Fit stand mixer with whisk and whip egg whites on medium-low* speed until foamy, 2 to 2 1/2 minutes. Increase speed to medium-high and whip until stiff peaks just start to form, 2 to 3 minutes. Using rubber spatula, fold egg yolk mixture into egg whites until no white streaks remain. Heat remaining 1 tsp. butter in 12-inch ovensafe nonstick skillet over medium-high heat, swirling to coat bottom of pan. When butter foams, quickly add egg mixture, spreading into even layer with spatula. Remove pan from heat and gently sprinkle filling and Parmesan evenly over top of omelet. Transfer to oven and cook until center of omelet springs back when lightly pressed, 4 1/2 minutes for slightly wet omelet and 5 minutes for dry omelet. Run spatula around edges of omelet to loosen, shaking gently to release. Slide omelet onto cutting board and let stand for 30 seconds. Using spatula, fold omelet in half. Cut omelet in half crosswise and serve immediately.

Asparagus and Smoked Salmon Filling

1 tsp. olive oil
1 shallot, sliced thin
5 oz. asparagus, trimmed and cut on bias into 1/2-inch lengths
Salt and pepper
1 oz. smoked salmon, chopped
1/2 tsp. lemon juice

Heat oil in 12-inch nonstick skillet over medium-high heat until shimmering. Add shallot and cook until softened and starting to brown, about 2 minutes. Add asparagus, pinch salt, and pepper to taste, and cook, stirring frequently, until crisp-tender, 5 to 7 minutes. Transfer asparagus mixture to bowl and stir in salmon and lemon juice.

Mushroom Filling

1 tsp. olive oil
1 shallot, sliced thin
4 oz. white or cremini mushrooms, trimmed and chopped
Salt and pepper
1 tsp. balsamic vinegar

Heat oil in 12-inch nonstick skillet over medium-high heat until shimmering. Add shallot and cook until softened, about 2 minutes. Add mushrooms and 1/3 tsp. salt and season with pepper to taste. Cook until liquid has evaporated and mushrooms begin to brown, 6 to 8 minutes. Transfer mixture to bowl and stir in vinegar.

Artichoke & Bacon Filling

2 slices bacon, cut into 1/4-inch pieces
1 shallot, sliced thin
5 oz. frozen artichoke hearts,
thawed, patted dry, and chopped
Salt and pepper
1/2 tsp. lemon juice

Cook bacon in 12-inch nonstick skillet over medium-high heat until crisp, 3 to 6 minutes. Transfer bacon to paper towel-lined plate. Pour off all but 1 tsp. fat from skillet. Add shallot and cook until softened, about 2 minutes. Add artichokes, 1/3 tsp. salt, and pepper to taste. Cook, stirring frequently, until beginning to brown, 6 to 8 minutes. Transfer artichoke mixture to bowl and stir in bacon and lemon juice.