Heat a non-stick skillet over medium heat. Add butter or olive oil, then place sliced mushrooms in a single layer without crowding. Cook for 3 to 4 minutes without frequent stirring until golden brown with slightly crisp edges.

Stir in minced garlic and cook for 30 seconds until fragrant. Add fresh spinach and toss gently for 1 minute until fully wilted. If using pre-drained frozen spinach, it will wilt faster.

Lower heat to medium-low. In a small bowl, whisk together eggs, milk or cream, salt, and pepper until fully blended. Pour the mixture over the vegetables.

Using a wide spatula, fold the eggs gently from the outside edges toward the center every 20 to 30 seconds. Cook for 2 to 3 minutes until eggs look glossy and just barely set at the edges but still soft in the center. If using cheese, add now and fold in. Remove from heat while slightly underdone and serve immediately.