Heat the oven to 180°C/Gas 4. Cut the onion in half and slice finely. Cut the red and yellow peppers into long strips, discarding the core and seeds.
Heat the 1 tbsp oil in a fry pan, and fry the onion and peppers for 10 minutes until they start to soften. Slice the courgettes and tomatoes into rings.
Arrange the onion and peppers in the base of a lightly oiled baking tin or gra