Place a large, non-stick frying pan with an ovenproof (metal) handle over a high heat, add the oil and fry the onion, courgette and peppers until tender. Add the garlic and fry for a further 1 minute.
Whisk the eggs in a large bowl until just combined, then add the contents of the frying pan and gently mix through until coated. Season with sea salt and ground black pepper.
Place the frying pan back over a medium–high heat and add the butter, melting until it foams. Pour the egg and vegetable mixture into the pan and evenly distribute the vegetables. Sprinkle over the cheese.
Cook for 4 minutes until the bottom of the frittata is golden brown, then place the pan under a hot grill to puff up for a further 3–4 minutes until golden on top.
Remove the frittata from the pan and cut into quarters on a chopping board. Serve with a little side salad for a quick and tasty lunch or dinner.