If making homemade meatballs: In a large bowl, combine ground beef, panko breadcrumbs, egg, minced garlic, Italian seasoning, red pepper flakes, and salt. Mix gently with your hands for 30-45 seconds until just combined - overmixing makes tough meatballs. Form into 20 evenly-sized meatballs (about 1 to 1.5 inches in diameter).

Heat olive oil in a large skillet over medium-high heat until it shimmers (about 2 minutes). Once the oil moves freely across pan, add meatballs in a single layer. Turn every minute to brown all sides evenly, about 6-8 minutes total. Meatballs should be deep golden brown.

Reduce heat to low and let meatballs rest for 1 minute. Carefully add marinara sauce to prevent splatter. Cover skillet with lid and simmer for 15 minutes, or until internal temperature reaches 160°F.

If using frozen meatballs: Heat marinara sauce in large skillet over medium heat until simmering. Add frozen meatballs, cover, reduce heat to medium-low, and cook for 25 minutes until heated through.

During the last 5 minutes of meatball cooking time, position oven rack 8-10 inches below broiler element. Place hot dog buns on a baking sheet and preheat broiler.

Using a slotted spoon, place 3 meatballs in each bun. Spoon 2-3 tablespoons of marinara sauce over top and sprinkle with 3 tablespoons shredded mozzarella cheese.

Place baking sheet under broiler for 2-3 minutes, watching carefully, until cheese melts and bubbles and bun edges toast lightly. If cheese browns too quickly, move rack down 2 inches. Serve immediately.