To make the topping, rub the butter into the flour until it resembles very coarse breadcrumbs, then stir in the rest of the topping ingredients apart from the egg, cover and chill in the fridge for 30 minutes
To make the filling, warm the olive oil in a very large saucepan over a medium-low heat; add the onions, leeks, thyme and a generous pinch of salt and sauté until the onions and leeks are soft, about 15-20 minutes; add the garlic and sauté for a further couple of minutes
Place the spinach in another pan and add 2 tablespoons of water, bring to the boil, stirring frequently; drain well in a colander; then add to the onion mixture, mix well, cooking for a couple of minutes; tip into a large bowl, remove the thyme and mix in the parmesan, nutmeg and zest then season; crumble in the ricotta, leaving some large pieces; gently mix through the spinach; place in a large baking dish (about 20cm x 30cm)
Preheat the oven to 200C/Gas6; mix the egg into the crumble mixture with a knife, then scatter it over the spinach and bake for 35-40 minutes until the crumble is piping hot and golden brown; scatter over a few extra thyme leaves and serve
6 servings