Start by heating the olive oil and butter in a skillet over medium heat. Once hot, add the thinly sliced onions. Sprinkle them with salt, and stir occasionally. The key here is patience. Keep cooking them for about 20-30 minutes until they're beautifully golden brown. Towards the end, add the balsamic vinegar and thyme, letting them cook for a minute longer. Set these aside; they’ll be your sweet, caramelized topping.

In a mixing bowl, combine the ground beef, Worcestershire sauce, onion soup seasoning, and kosher salt. Mix until just combined. Don’t overdo it; you want a tender burger!

Divide the beef mixture into equally sized portions, typically around six ounces each. Shape them into patties, slightly larger than your burger buns because they'll shrink while cooking.

Preheat a grill or skillet over medium-high heat. Once it's hot, place the patties on it. Cook for about 4-5 minutes on one side. Flip, and then add a slice of cheese to the top after about another 2 minutes. If you want crispy bacon, you can cook that while the burgers are grilling.

While the burgers finish up, toast your burger buns on the grill or in a toaster. This step adds texture and prevents them from getting soggy when you pile on all the toppings.

Now for the fun part! On the bottom of each toasted bun, layer lettuce and tomato slices. Next, place the patty with melted cheese and top it off with the caramelized onions. If you like, drizzle some dijonnaise on top. Close it up with the bun lid. Voilà!