Preheat the oven to 400 degrees F. Using a serrated knife, slice off the top quarter of each tomato and reserve the tops. Scoop out the pulp from inside each tomato and transfer it to a food processor. Add 2 tablespoons olive oil and the cayenne to the food processor and process until smooth; pour into a 9-by-13-inch baking dish.
Heat the remaining 2 tablespoons olive oil in a large skillet over medium-high heat. Add the sausage, herbes de Provence, bell pepper, onion, diced celery and garlic. Cook, stirring, until the sausage browns, about 5 minutes. Transfer the sausage mixture and bread cubes to the food processor and pulse until chopped. Pack the tomatoes with the sausage-bread mixture so the stuffing is domed on top. Put the stuffed tomatoes in the baking dish on top of the sauce.
Place the basil, walnuts, cheese and celery leaves in the food processor and chop. Sprinkle onto the tomatoes. Put the tomato tops, cut-side down, between the stuffed tomatoes. Bake until the stuffing is golden, about 20 minutes. Cover each tomato with a top and serve with the sauce.