Whisk the eggs, milk, salt, pepper, chili powder, and cumin in a bowl.
Heat an 8-inch nonstick skillet over medium-low heat.
Swirl in butter or oil across the pan.
Cook bell pepper and onion for 2 minutes.
Stir in tomato for 30 seconds, then transfer filling to a plate.
Lower heat slightly, then pour in the egg mixture.
Tilt the pan so the eggs spread evenly.
Pull cooked edges inward with a spatula.
Let loose egg run toward the edges.
Add cheese and cooked filling over one half.
Cook until the top looks barely set.
Fold the plain half over the filling.
Slide the omelette onto a plate.
Top with salsa, cilantro, avocado, olives, or jalapeño.