(Caramelized onions)

1. Cut the onions in half and slice thinly. Heat butter and oil together in a large skillet over medium-low heat.
2. Add the sliced onions, salt, pepper, and thyme sprigs.
3. Cook the onions slowly, stirring occasionally, until deeply browned and caramelized, about 45 minutes.
4. Remove the thyme sprigs, raise the heat to high, and add the balsamic vinegar.
5. Cook, stirring, until all the vinegar has evaporated. (Can be made several days ahead.
6. Store in a covered container in the regrigerator and reheat in the microwave or on the stovetop before using.)

(Spicy mayonnaise)

1. Mix the mayonnaise and Sriracha together until fully combined. Refrigerate until ready to serve.

(Make the burgers)

1. Prepare the grill for direct heat and preheat to medium-high.
2. Combine the ground meats. Divide into 6 equal portions and shape into patties about 3/4 inch (2 cm) thick.
3. Make a deep impression with your thumb in the center of each patty.
4. Brush both sides of the patties with oil and season to taste with salt and pepper.
5. Grill for 4 minutes, flip the burgers and grill for another 4 to 5 minutes for medium-rare.
6. Brush the insides of the hamburger buns with melted butter and grill for 1 to 2 minutes.
7. To serve spead the botom bun with some of the spicy mayonnaise, top with letuce tomato, a burger patty, and some caramelized onions.