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Preheat the oven to 190C/gas 5/fan 170C and butter a shallow 2-litre dish. Tip the spinach into a colander in the sink and pour a kettleful of boiling water over it. Leave it to wilt while you make the cheesy base. (Or microwave the spinach according to the packet instructions.)
Break the eggs into the bowl of a food processor, pour in the milk, add the mustard and 1 tsp salt. Tear in the bread, crusts and all, then whizz together until smooth. Tip the mixture into a large bowl and grate in three-quarters of the cheese.
Squeeze the spinach, to get rid of all the water, then stir it into the cheese mixture and tip into the dish. Grate over the remaining cheese, top with the tomato clusters and bake for 30-35 minutes until risen and golden. Cool and allow to settle a little before serving.