Make the onions:
Heat large skillet over MED heat, melt butter. Slice onions into half rings and add to the hot skillet. Reduce heat to MED-LOW, add sprigs of thyme and cook for 40 minutes, stirring often. Onions will soften and eventually turn a caramel brown color.
Make the bacon:
Sprinkle pepper and cayenne pepper over the bacon slices and cook as desired. (Baking is my preferred method).
Make the mayo:
In a small mixing bowl, whisk together mayo, A1 steak sauce and cracked black pepper. Set aside (or refrigerate if making ahead of time).
Make the burgers:
In a large mixing bowl, add ground beef, beef bouillon, dried minced onion, celery seed, dried parsley, dried thyme, and black pepper. Using your hands (the two best kitchen tools you have!), mix together until seasonings are evenly dispersed. Using the side of your hand, draw a line dividing the mixture into 4ths. Divide each section into 3 parts of meat and roll into a ball. You'll end up with 12 evenly-sized balls.
Heat a griddle or large skillet over HIGH or MED-HIGH heat. Coat the surface with butter and add several balls of meat (several inches apart). Let them cook for 2 minutes, undisturbed. Carefully flip the balls over and immediately use a wide spatula or grill press to smash the burgers down and flatten them out. Cook for 2 minutes, then flip over again.
Sprinkle with a little shredded cheese and continue to cook until cheese is melted.
Remove to a plate, lightly covered with foil to keep them warm, and continue to cook remaining burgers.
Make the sliders:
Build your slider by adding a bit of arugula lettuce to the bottom of a slider bun. Add the burger on top of the lettuce. Top burgers with caramelized onions. Break a cooked bacon slice in half and place the two halves on top of the onions. Slather A1 mayo on the top slider bun and top the slider.