Peel and slice onions to half moons. Cook in oil over a medium heat in a 20cm (8in) oven proof frying pan. If your pan isn’t oven proof you can use a spring form pan for the baking part.
Preheat your oven to 200C (400F). When the onions are soft and golden, remove from the heat. Add thyme (if using), eggs, half the cheese and stir to combine well. Top with remaining cheese.
Bake for 10-15 minutes or until frittata is puffy and golden brown. Divide between two plates and serve with salad leaves on top.