Mix mayonnaise, vinegar, pesto and cumin in a mixing bowl and set aside.
Heat griddle or grill to high heat.
Form meat into 6 patties, then sprinkle with seasoned salt and place on grill. Sear until brown, about 2 minutes, then flip and repeat for other side. Reduce heat to medium and continue cooking until desired temperature is reached.
Heat butter in skillet on medium heat and add onion. Cook, stirring occasionally, until caramelized and a golden-brown color.
Place mozzarella on patties and let it melt. (It can help to put the cheese on a minute or two before the meat is totally cooked, to keep the moisture in and not overcook the meat.)
Add the onion on top of the cheese and place patties on bottom buns. Top with liberal amount of sauce, 3 tomato slices and the bun tops.
Serve while hot.
Make sauce ahead of time so the burgers are not getting cold while you finish dressing them. To have the best burgers, look for homemade-style buns from a local bakery. Use fresh, high-quality ingredients and produce for best results.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.