Combine the crème fraîche, mayonnaise and sweet mustard. Season to taste with salt, pepper and sugar. Add the cabbage and carrots and stir until everything is covered. Let stand for about an hour.
Form the ground beef into 4 patties of approx. 125 grams each and season well with salt and pepper. Brown the patties in some oil on each side. Brush the patties with a generous amount of barbecue sauce. Pop them in the oven for 6-8 minutes at 200°C. Then top with a Castello Blue Cheese Slice and give them another couple of minutes until the cheese has melted.
Warm the buns. Place a spoonful of coleslaw on the bottom half of the bun. Then add a patty and a slice of tomato and finish off with the top half of the bun spread with barbecue sauce.