My Grandmother's Quick Spinach & Mushroom Omelette

Ingredients

• 4 large eggs (240 ml)
• 1 cup fresh spinach, chopped (30 g)
• 1 cup mushrooms, sliced (70 g)
• 1/4 cup milk (60 ml)
• 1 tablespoon unsalted butter (14 g)
• Salt and pepper, to taste
• Fresh parsley, chopped, for garnish (optional)
• Grated cheese (e.g. feta or cheddar), optional