Gently mix 1 pound beef, 1 pound pork, 1/4 cup minced onion, 4 cloves minced garlic, 3 tablespoons paprika, 1 tablespoon cumin, 2 teaspoons salt, 1 teaspoon pepper, and 1/2 teaspoon cayenne in a large bowl. Roll mixture into 1 1/2-inch diameter meatballs and place on a baking sheet.
Heat oven to 300°F. Heat a large cast iron skillet over medium heat and add flatbreads one at a time. Cook until toasted and brown, about 2 minutes per side. Wrap flatbreads in foil and place in oven to keep warm.
Heat 2 tablespoons olive oil in same cast iron skillet over medium heat. Add onions and season with salt and pepper. Cook until caramelized and brown, lowering the heat if they are starting to brown too quickly before becoming tender, about 15 minutes. Transfer to one side of a baking sheet and place in oven to keep warm.
Heat remaining 2 tablespoon of oil in the same large skillet over medium heat. Brown half of the meatballs on all sides until just cooked through (about 8-10 minutes), then place them on the baking sheet with the onions and return baking sheet to oven. Repeat with remaining meatballs, adding more oil if skillet seems dry.