Preheat the oven to 190°C. Grease and line the base of a 20 cm sandwich tin.
Heat the oil in a deep heavy-based frying pan. Gently cook the onions for 5 minutes until transparent, then add the capsicums and thyme. Add the garlic and cook for 20 minutes over a moderate heat, stirring occasionally.
Add the olives and cheese to the vegetables, remove the thyme sprigs, if using fresh, and take the pan off the heat.
Season the eggs with pepper, then stir into the vegetables and mix. Pour into the tin. Bake for 35 minutes until golden.
Remove from the oven and rest for 3 minutes. Turn out onto a plate, then flip right side up. Cut into wedges to serve.
If you have any leftover fetta cheese, dice it finely with some stoned olives, drizzle with a little olive oil and chopped fresh herbs and use to fill hollowed-out cherry tomatoes. Serve as a nibble with drinks.