Heat oil in a large frying pan over medium-high heat. Add tomatoes and sauté until soft. Add spinach and cook, stirring until wilted. Transfer vegetables to a sieve and use the back of a wooden spoon to remove excess liquid, then arrange vegetables in a square 24cm baking dish.

Return frying pan to the heat and cook chorizo slices for 2 minutes on each side. Arrange in casserole dish along with bread pieces.

In a bowl, whisk together eggs, milk, mustard and cheeses, then pour over vegetables in the casserole dish. Cover with aluminium foil and refrigerate overnight.

Remove casserole dish from the refrigerator. Preheat oven to 180°C.

Bake strata uncovered in preheated oven for 1 hour or until a skewer inserted into the middle comes out clean. Serve immediately.