In a medium bowl, whisk together eggs, milk, salt, and pepper.
In a 10-inch nonstick skillet, melt butter over medium heat. Add bell pepper, onion, mushrooms, and tomatoes. Cook for 5 minutes, or until vegetables are tender.
Pour egg mixture into the skillet and cook for 2 minutes, or until the bottom is lightly golden. Sprinkle cheese and herbs over the omelet. Using a spatula, lift up the edges of the omelet and tilt the pan to let uncooked egg mixture flow underneath. Cook for another 2 minutes, or until the omelet is lightly golden.
Gently flip the omelet and cook for 1 minute, or until the omelet is cooked through.
Slide the omelet onto a plate and enjoy!