For caramelized onions: In medium saucepan over medium-low heat, melt butter and olive oil. Add sliced yellow onion and sauté, stirring occasionally for 10 minutes. Add sugar and continue cooking for 20-30 minutes until onions are caramelized and golden in color. You might need to adjust heat throughout the process to ensure that the onions don't burn. 

For cheeseburgers: Place ground beef in large bowl along with minced garlic, Worcestershire sauce, salt and pepper. Carefully mix everything in and divide the meat into 4 equal portions. Shape meat into burger patties and set on a dinner-sized plate. Butter each half of the hamburger buns with 1/2 tablespoon of unsalted butter and sprinkle evenly with garlic salt. Place buns on a separate plate.

Fire up the grill to medium-high heat and let it heat for at least 5 minutes. When grill is nice and hot, add patties and grill 2-3 minutes each side. Top each burger with a thick slice of Wisconsin mozzarella cheese and cover grill so cheese melts, about 1-2 minutes. When you put cheese on the burgers, it's also a great time to put buns, buttered side down, on the grill so that they get nice and toasty! 

For assembly: Mix mayonnaise and mustard in small bowl. Slather each bun with desired amount of mustard mixture. Add burger, a handful of fresh arugula and tomato slices if desired and top with 1/4 of the caramelized onions.