Spinach and potato frittata

Ingredients

• 3 teaspoons olive oil
• 1 small onion, finely chopped
• 1 clove garlic, finely chopped
• 4 medium eggs
• 1 tablespoon skimmed milk
• 160g frozen spinach, thawed
• 20g reduced-fat mature Cheddar cheese (or similar), grated
• Freshly ground black pepper
• 240g cooked potato (2 small baked potatoes or 6 new potatoes), cubed or sliced
• ½ courgette, thinly sliced (optional garnish)