Peel the garlic and press it through a garlic press. Mix with egg yolk and vinegar until creamy. Add 10 tablespoons of oil, first drop by drop, then in a thin stream, stirring constantly.
Season with salt and pepper, put aside. Peel and finely chop the onion. Wash the rosemary, dab dry, put something aside for garnishing. Chop the rest finely. Wash and clean the tomatoes and halve half of the tomatoes.
Drain the olives. Heat 1 tablespoon of oil in a pan. Brown the onion and rosemary. Add the tomatoes, season with salt and pepper and braise over a low heat for 4-5 minutes. Wash the fish, dab dry, season with salt and turn in flour.
Heat 3 tablespoons of oil in a frying pan, fry the sardines in portions while turning for about 5 minutes. Drain on kitchen paper. Mix the olives with the tomatoes, season with salt and pepper.
Serve sardines with tomatoes and aioli Garnish with rosemary. Serve with baguette bread.