1. Combine all the ingredients for the mustard sauce and refrigerate for a few of hours – preferably overnight.
2. Roast the onion slices until blackened on a very hot and dry pan. Divide them into onion rings and set aside.
3. Form the meat into 4 patties and season with salt and pepper.
4. Heat a grill pan until smoking hot. Fry the patties for about 3 minutes on each side – set them aside to rest.
5. Split the buns and warm or toast them. Place a slice of Burger Blue cheese on the patties and spread the buns with mustard sauce, then top the buns with lettuce, patties, roasted onions and tomatoes.
6. Finish with the top bun and serve with pickles, fries with a sprinkle of thyme and extra mustard sauce.