Set a large skillet over medium heat and melt 3 tablespoons of butter. Slice the yellow onions into half rings and add them to the hot skillet. Reduce heat to medium-low and lay the fresh thyme sprigs on top of the onions. Cook, stirring often, for about 40 minutes until the onions are soft and caramelized to a deep brown color. Remove the thyme sprigs before using the onions for assembling the sliders.

While the onions are caramelizing, lay the bacon strips out on a baking sheet. Sprinkle the bacon with a pinch of black pepper and a pinch of cayenne pepper. Bake in a preheated oven at 400°F (200°C) for about 15-20 minutes, or until crispy, or cook using your preferred method. Once cooked, transfer the bacon to a paper towel-lined plate to drain excess fat. I prefer oven-baking bacon for evenly crispy results and less mess.

In a small mixing bowl, whisk together the mayonnaise, A1 steak sauce, and a pinch of cracked black pepper until thoroughly combined. Refrigerate if making ahead. I like to make extra A1 mayo for dipping fries or spreading on other sandwiches too!

In a large mixing bowl, combine the ground beef or brisket, beef bouillon cubes (crumbled or crushed), dried minced onion, celery seeds, dried parsley, dried thyme, and 1/4 teaspoon ground black pepper. Using your hands, mix the ingredients until the seasonings are evenly blended into the meat. Divide the mixture into 12 even-sized balls. Heat a griddle or large skillet over high or medium-high heat and coat the surface with 2 tablespoons of butter. Place several meatballs onto the hot surface with space between each. Let them cook undisturbed for 2 minutes, flip, then immediately smash the burgers down with a spatula or grill press. Cook another 2 minutes, then flip again. Top each patty with some shredded Gruyere cheese and cook until the cheese is melted. Transfer cooked patties to a plate and cover lightly with foil while you cook the rest.

To build your sliders, add a bit of arugula to the bottom half of each toasted slider bun. Place a burger patty with melted Gruyere cheese (from Step 4) on top of the arugula. Spoon caramelized onions (from Step 1) generously over each burger. Break a cooked bacon slice (from Step 2) in half and lay the pieces over the onions. Spread A1 mayo (from Step 3) on the top half of each bun, then cap the sliders and serve immediately. For a fresh touch, I like to warm the buns in the oven for a couple of minutes before assembling.