Preheat the oven to 350° F. To make the tomato sauce, melt the butter over medium heat in a large flameproof casserole or Dutch oven. Add the bacon and cook until golden brown. Add the onion and carrot and cook for 3 minutes. Add the tomato paste, stir well and cook for another 2 minutes. Sprinkle with the flour and place in the oven for 5 minutes. Remove and stir well until the flour disappears, then add the tomatoes, bouquet garni and garlic. Cook on the stove for 5 minutes, stirring well, then add the stock or water, bring to a boil, stirring, and cook for 2 minutes. Cover and bake for 1 hour.

Bring a large saucepan of salted water to a boil. Add a splash of oil to stop the pasta from sticking and cook the shells according to the manufacturer's instructions. Drain and drizzle with a little olive oil. Arrange on a damp towel.

Lightly oil a large baking dish. In a bowl, mix together the ricotta, Parmesan, eggs, herbs and nutmeg and season with salt and pepper. Spoon the mixture into a pastry bag with a plain nozzle, fill each shell and arrange in a single layer in the prepared dish.

Strain the tomato sauce through a strainer, pressing well in order to extract as much liquid as possible. Discard the solids. Bring the sauce back to a boil and skim if necessary. Simmer for 20 minutes, or until the sauce thickens. Pour over the stuffed shells. Sprinkle the mozzarella over the sauce and bake for 30 - 40 minutes, or until the cheese is melted and browned.