Cheesy Italian Meatball Sandwich

Ingredients

• 1 onion (large, minced)
• 3 tablespoons olive oil
• 4 cloves garlic (minced)
• 1 slice prosciutto (cut ¼" thick, then diced)
• 2 slices pancetta (cut ¼" thick, then diced)
• 1 pound ground round
• 1 pound mild Italian sausage (ground)
• 2 cups seasoned bread crumbs (Italian-)
• 2 jumbo eggs
• 1/2 cup Italian parsley (chopped)
• 1/8 cup fresh basil (chopped)
• 1/2 cup grated parmesan cheese (finely)
• 1/2 cup grated pecorino cheese (finely, you can also use asiago)
• 2 tablespoons italian seasonings (dried or fresh Italian herbs: oregano, sage, rosemary, basil, etc.)
• 1 1/2 teaspoons fennel seeds (whole)
• 2 jars marinara sauce (homemade or store-bought, plain)
• 2 tablespoons italian seasonings (dried or fresh, extra if needed to suit your taste preferences)
• 1/4 teaspoon red pepper flakes
• 1 teaspoon sea salt (freshly cracked)
• 2 breads (hefty, Ciabatta or large hoagie/submarine buns)
• olive oil
• 1 large garlic clove (cut in half)
• 2 fresh mozzarella (balls of, sliced about ⅛ - ¼" thick)