1. Boil the vinegar, sugar and water in a saucepan. Add the red onions and let the pickling juice cool in the refrigerator.
2. Cut the carrots into very thin strips with a peeler. Heat some oil in a saucepan. Test the temperature with a small piece of bread – the bread should turn golden.
3. Roast the carrots in the hot oil for about 3 min. Remove them with a slotted spoon and let them drain. Place the carrot fries on some paper towels and sprinkle with sea salt.
4. Stir the ground meat with salt and freshly ground black pepper. Form the meat into 4 burgers (about 10 cm in diameter).
5. Melt the butter in a saute pan until golden. Fry the steaks about 1½ min. on each side over medium heat for about 2 min. Cover the pan with a lid, remove it from the heat and let the burgers rest.
6. Warm up the buns in the oven and cut them in half. Spread them with chili mayonnaise and top the bottom bun with lettuce and sprouts.
7. Add the burgers and top with a slice of Castello® Burger Blue and the pickled red onions. Finish with the top bun and serve with the carrot fries.