Whisk up 2-3 eggs in a small mixing bowl until thoroughly blended.
(Or pulse for a short time in a blender.)
Set eggs aside.
In a heavy 9" non-stick or cast-iron skillet, over medium heat, heat up a small amount of olive oil.
Add diced/chopped and sliced vegetables to skillet, sauteeing until just softened.
(I used red onion, red pepper, broccoli, spinach and mushrooms.)
Sprinkle with salt and pepper. Mix thoroughly.
Remove seasoned vegetables from skillet and set aside in a small bowl.
And butter to the skillet.
Heat until sizzling.
Pour eggs into the skillet.
Eggs should begin cooking immediately. (If they don't, your skillet isn't hot enough.)
With a flexible spatula, begin loosening the edges of the omelet, all the way around the pan.
Pull the edges away from the pan and tilt the skillet.
Rotate the pan so that the uncooked eggs in the middle slide underneath the omelette where the edges are pulled away.
Keep pulling the edges back and tilting the uncooked eggs until the top is shiny and the eggs are cooked around the edges.
Sprinkle with grated cheeses, as much as you like.
(I used a Mexican shredded cheese blend.)
Sprinkle roasted or sauteed veggies on top of the grated cheeses.
(I used red onion, red pepper, broccoli, spinach and mushrooms.)
Sprinkle cooked meats on top.
(I used chopped bacon.)
Sprinkle with more cheese if you like.
With a thin spatula, gently loosen the edges of the omelet and underneath.
If the bottom of the omelet isn't yet turning golden brown, allow it to cook for another minute or two over the heat.
Then fold the omelet over in half, and let sit for another minute or so over the heat to allow the cheese inside to melt.
Serve immediately, hot.
Optional: garnish with fresh parsley.