Cheesy Spinach Frittata - Slender Kitchen

Ingredients

• 8 large eggs
• 1/3 cup skim milk
• 3/4 cup shredded cheddar cheese (I used reduced fat)
• 1/2 teaspoon kosher salt
• 1/4 teaspoon freshly ground black pepper
• 1 tbsp olive oil
• 5 cups baby spinach
• 1/4 onion, sliced very thin
• 2 tbsp Parmesan cheese