Heat a very large, heavy-based saucepan over a medium heat, then add the bacon and dry-fry for 3–4 minutes or until beginning to colour. If the bacon is very lean, fry in 1 tablespoon olive oil. Add the shallots and garlic and fry for 2–3 minutes or until softened.
Stir in the white wine, tomatoes and thyme and season with salt and pepper to taste. Bring to the boil, then reduce the heat and simmer for 20 minutes or until the sauce has thickened.
Add the mussels and stir to coat, then cover with a tight-fitting lid and cook for 4–5 minutes, or until the mussels have opened, stirring once. Discard any that remain closed.
Stir in the crème fraîche, if using, and serve in deep bowls garnished with thyme sprigs.