In a sauté pan warm the olive oil and butter on medium high heat. Slowly add in the onions in batches; you do this by cooking down and softening a portion then adding more on top until you have cooked all of your onions. Add in the bay leaf. Sprinkle the onions with sugar and season with salt. Cook for about 15-20 minutes or until the onions are golden brown in color. Remove from the heat and reserve.
In a bowl, add in the veal and the beef, the egg, white wine, Worcestershire sauce, garlic powder, bread crumbs, salt, pepper, thyme and parsley. Combine until all the ingredients are mixed well into the meat. Divide the meat into six parts and form your burgers. Using your fingers create an indent in the center of the burger so the middle is thinner than the edges for more even cooking.
Cook the burgers in a cast iron skillet on medium high heat (or a stainless steel skillet or a grill, as you wish) warmed with some olive oil for about seven to eight minutes per side. When you flip them place the sliced Gruyère cheese on top and let it melt while the burger finishes to cook through.
Build your burger by layering the bun, the lettuce, meat, caramelized onions and then brush the top bun with some Dijon mustard and there you have it!