1 tablespoon olive oil 4 large sweet onions, sliced 1 tablespoon balsamic vinegar 1 pound 96 percent lean ground beef 1/2 teaspoon salt 1/4 teaspoon ground black pepper 1/2 teaspoon cumin 1/4 teaspoon cayenne pepper 1 teaspoon smoked paprika 1/2 teaspoon garlic powder 1 tablespoon honey 1 tablespoon tomato paste 1 teaspoon Dijon mustard 3 tablespoons apple cider vinegar 1 tablespoon soy sauce 4 standard or 8 slider whole-grain burger buns In a large skillet over medium, heat the olive oil. Add the onions and saute until deep brown, about 20 to 25 minutes. If the skillet gets too dry, add 1 to 2 teaspoons of water as needed.