A reliable upscale option a short walk from the cathedral, D'Hoogh has been one of the standard recommendations in Mechelen for decades. The cooking is classical Belgian-French — fish dishes, game in season, the local Mechelse chicken done several ways — served in a handsome dining room with proper service.
It is not cheap, but the multi-course set menus give a good sense of what Flemish fine dining looks like outside the big-city restaurants. Reservations strongly advised at weekends.