Heat butter in a medium nonstick pan over medium-high heat. Sauté bell peppers, spinach, and tomatoes until tender, about 3–4 minutes.

Crack eggs into a bowl, add milk, salt, and pepper. Whisk vigorously until smooth and frothy.

Lower heat to medium-low and melt another pat of butter, swirling to coat the surface.

Pour the egg mixture into the pan, tilting to spread evenly. Cook 1–2 minutes until edges set.

Sprinkle the sautéed veggies evenly over one half, then top with grated cheddar.

Using a spatula, carefully fold the empty half over the fillings. Slide onto a warm plate and let rest 30 seconds.

Sprinkle with Parmesan (if using) and serve immediately.