In a large bowl, combine the ground beef, breadcrumbs, white onion, Worcestershire sauce, sweet chilli sauce, egg, salt and pepper into a beef paste. Form the beef mixture into 8 patties around ½ inch thick and 4 inches wide. Set aside on a piece of parchment paper.
Place a large skillet/frying pan on high heat and add in ½ tablespoon vegetable oil. Once hot, add in 4 beef patties and fry on each side for 1 minute, pressing down with the back of a spatula to sustain the size and shape of the burger patty. Once you've fried on each side, reduce the heat to medium. Add the cheddar cheese onto the burgers and place on the lid of the skillet. Cook for a further minute. Then remove the burger patties and set aside. Repeat with the 4 other patties, finishing with the burger cheese. Optionally keep the burger patties warm in a preheated oven at 160c.
Meanwhile, prepare your mushroom sauce. Place a pan on medium heat and drizzle with vegetable oil and butter. Once melted, add in the garlic, mushrooms, and peppercorns. Fry for a couple of minutes, then add in the balsamic vinegar. Fry for another minute, stirring frequently.
Add in the single cream and beef stock. Stir to combine. Once the sauce is hot, reduce the heat to low. Taste and season with salt and pepper accordingly.
Toast your brioche buns, and now it's time to assemble! Dollop a teaspoon of your mayonnaise, ketchup and jalapeño relish on the base of your brioche bun. Top with the two cheesed patties and top with more relish. Repeat for all 4 burgers.
Serve up your burgers with your mushroom and peppercorn sauce! Enjoy!
Notes
Step-by-Step Photos: My blog post includes helpful step-by-step photos to help guide you through making this recipe - so scroll up for more!