Combine the mince, red onion, eggs, Cajun seasoning and breadcrumbs in a large bowl then mix well, until the mixture is sticky. Form into four patties, then refrigerate for 1 hour.
Place the zucchini and eschalots in a sterilised jar. In a saucepan, put the vinegar, spices, sugar and fine salt and boil rapidly for 5 minutes. Set aside to cool, then strain onto the zucchini. Stand for 1 hour, or overnight.
Sprinkle the burger patties with cooking oil spray and cook over a hot grill for 8 minutes, turning regularly. Top each pattie with 2 cheese slices to melt gently.
Top the bagel bases with lettuce and tomato, then add the patties, pickled zucchini and smoky barbecue sauce.