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Prepare to the end of step 5 the day before, keeping the filling and topping separately in the fridge. Allow an extra 10-15 mins when cooking from chilled.
Preheat the oven to 190°C, fan 170°C, gas 5.
Add the red onion and peppers and stir-fry for 3 minutes, then add the courgette and garlic and cook for a further 2 minutes.
Stir in the dried herbs, tomatoes and water or stock. Bring to a simmer, season and cook, partially covered, for 5 minutes. Add the spinach, re-cover the pan and cook until just wilted, then tip the ratatouille into a large baking dish, about 22cm x 30cm.
For the crumble topping, simply mix the dry ingredients together in a bowl with a little seasoning, then mix in the olive oil to help the crumble start to clump together.
Tear the mozzarella roughly over the ratatouille, then add the crumble topping. Bake in the oven for 25-30 minutes until the topping is crisp and golden brown, and the juices are bubbling around the edges of the dish. Leave for 10 minutes before serving.