Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Grease an ovenproof dish. Wash and slice the tomatoes. Cut the ciabatta bread into slices as well. Layer alternately in the baking dish.
Whisk the crème légère, milk and eggs. Season well with salt and pepper. Pour over the casserole. Bake in a hot oven for about 20 minutes. Wash the basil, shake dry, remove the leaves and sprinkle over the casserole.