Wash tomatoes and slice off tops. Scoop out pulp and seeds and some of the flesh. Salt insides and rest upside down on a sheet pan lined with paper towels to extract juices, about 20 minutes. Soak stale bread in the saved tomato pulp. Keep tomato flesh for your next salad. Meanwhile, chop shallots and mince garlic. Cube shiitake caps and prosciutto. In a hot frying pan, sweat prosciutto with olive oil. Add shallots and sauté for a few minutes. Add garlic and cook until fragrant – throw in mushrooms and cook down for 4 min on high; cool. In the meantime, season ground meat with salt and pepper and set aside. In a large bowl, combine mushroom mixture, the soaked bread, the egg and meat. Fill up tomatoes generously and top with a chunk of butter.
Bake at 350ºF/180ºC for 25 min without the tops. Add tops and bake for 25 to 30 minutes more. Drizzle tomatoes with some lemon juice, season with fleur-de-sel and serve. Enjoy!