1. Place the tuna steaks in a large shallow dish; drizzle with lemon juice and olive oil; sprinkle with herbes de Provence, paprika, and season with salt and pepper. Let marinate for 30 minutes. Heat up your barbecue.
2. Peel the garlic and onions; wash the bell peppers, remove the seeds, and cut them into small pieces. Peel the tomatoes, cut them in half, squeeze them to extract the seeds and juice. Chop the onions and mince the garlic.
3. Heat a tablespoon of olive oil in a pan, sauté the onions and bell peppers lightly, then add the minced garlic, tomatoes, and bouquet garni; season with salt, pepper, and let simmer gently for 20 minutes.
4. Remove the tuna steaks from the marinade; lightly oil them, then place them on the grill and cook for 15 minutes, turning halfway through. Remove the bouquet garni from the sauce, pour it into the serving dish, place the tuna steaks on top, and serve immediately.