Whisk the eggs: In a bowl, beat eggs with milk, salt, and pepper until smooth and frothy.
Sauté veggies: Heat butter/oil in a skillet. Add onions, peppers, mushrooms, and tomatoes. Cook 2–3 minutes. Stir in spinach until just wilted. Remove and set aside.
Cook eggs: Pour whisked eggs into the skillet. Swirl to coat evenly. Cook 1–2 minutes until edges begin to set.
Add filling: Place sautéed veggies on one half of the omelet. Sprinkle cheese if desired.
Fold & finish: Gently fold omelet in half. Cook 1 more minute until set and cheese melts.
Serve: Slide onto a plate, garnish with fresh herbs, and enjoy warm.