Heat about 1 tablespoon oil in a large skillet over medium heat. Add the onions and cook until softened, about 2-3 minutes.
Add the beef to the skillet, breaking it up and cooking until most is browned, about 4-5 minutes.
Mix in the tomato paste and taco seasoning, stirring to combine. Add water or broth if the filling looks dry.
Continue cooking until the beef is cooked through and most liquid has evaporated, about 3-4 minutes. Season with salt if needed and set aside.
Place a tortilla on a work surface. Add mozzarella cheese, then beef filling, and more cheese on the top half. Fold the bottom half of the tortilla over and press to seal.
Transfer the tacos to a large baking sheet and repeat with remaining ingredients.
Heat about 3-4 tablespoons oil in a skillet over medium heat. Fry tacos in batches, about 1 minute per side or until golden brown.
Transfer cooked tacos to a wire rack and let sit for a moment. Garnish with fresh cilantro and serve hot.
Notes
Let the filling cool slightly before assembling to avoid soggy tacos. Fry in batches to maintain crispiness.