If using mushrooms or spinach, heat 1 tsp olive oil and 1 tsp butter in a medium nonstick skillet over medium heat. Add mushrooms and sauté 3–4 minutes until they release moisture and begin to brown. Season with 1/4 tsp salt and 1/4 tsp pepper. Add chopped spinach and cook 30–60 seconds until wilted. Transfer vegetables to a plate and set aside.

Warm meat and tomatoes (optional)

If using ham or bacon and tomatoes, add chopped ham or bacon to the skillet for 30–60 seconds just to warm through (if already cooked). Remove and combine with the sautéed vegetables. If using raw bacon, cook until crisp first and drain. Keep fillings warm and ready.

Grate the cheese and slice green onions and herbs. Have all fillings near the stove so you can assemble quickly once the eggs are ready.

Make the egg base

Crack 3 large eggs into a medium bowl. Add 2 tablespoons whole milk, 1 teaspoon kosher salt and 1 teaspoon freshly ground black pepper. Whisk vigorously until the mixture is uniform, slightly frothy, and pale in color (about 20–30 seconds). The milk makes a slightly creamier omelet but can be omitted.

Heat a 8–10 inch nonstick skillet over medium-low heat. Add 1 tablespoon unsalted butter and let it melt, swirling to coat the pan evenly. The pan should be hot enough that a drop of egg sizzles gently but not brown immediately.

Cook and fold omelet

Pour the beaten eggs into the center of the hot skillet. Immediately, with a heatproof rubber spatula or wooden spoon, stir the eggs gently in a circular motion for about 10–15 seconds to break up large curds, then stop stirring and let the eggs set for 20–30 seconds.

Tilt the pan and, using your spatula, push the set edges toward the center while tilting the pan to let uncooked egg run underneath. Repeat this process every 10–15 seconds until the eggs are mostly set but still slightly creamy on top (about 1–1.5 minutes total).

Turn off the heat. Sprinkle the fillings evenly over one half of the omelet: 1 oz grated cheddar, the sautéed mushrooms/spinach/ham/tomato as desired (use roughly 1/3 to 1/2 cup combined fillings), and 1 tablespoon sliced green onions.

Using the spatula, lift the empty half of the omelet and fold it over the filled half. Let the omelet sit in the warm skillet off the heat for 30–60 seconds so the cheese melts and fillings finish warming from residual heat.

Finish and serve

Slide the omelet onto a warm plate, seam-side down. Garnish with 1 teaspoon chopped fresh herbs and, if desired, 1 teaspoon hot sauce or salsa. Taste and add more salt or pepper if needed.

Serve immediately with toast, hash browns, or a side salad.