Heat the oil (or butter) in a small skillet over medium-high heat.

Add the mushrooms and cook until the mushrooms brown slightly. Don't crowd the mushrooms; cook in a single layer.

Reduce the heat to medium.

In a bowl, whisk the eggs, milk, nutmeg, Herbes de Provence, salt and pepper until frothy.

Pour the egg mixture into the skillet.

Cook over medium heat, loosening the eggs periodically with a spatula. Let some of the liquid spill over the side of the cooked eggs.

When the eggs are set (no longer liquid), flip the eggs to the uncooked side to finalize the cooking.

Spread the mushrooms over the top of the omelet and fold the cooked eggs in half to cover the mushrooms.

Remove the omelet to a waiting plate and garnish with sliced tomatoes.