Preheat oven to 350F and line a sheet pan with parchment paper.

Mix 1/2 cup panko breadcrumbs with 1/4 cup whole milk in a large bowl and let sit 5-10 minutes until soft.

Add 1 1/4 lbs ground beef, 1/3 cup chopped onion, 2 tsp minced garlic, 1 1/2 Tbsp minced parsley, 1 egg, 1/4 cup grated Parmesan, 1 1/2 tsp salt, and 3/4 tsp pepper to breadcrumb mixture.

Mix with spatula until just combined without overworking.

Form into 36 small meatballs using about 1 Tbsp portions each.

Space on sheet pan and bake 14-15 minutes until 165F internal temperature.

Grease a 9x13 pan and keep oven at 350F.

Heat 1 1/4 cups marinara sauce in a pot until hot, then toss cooked meatballs in the sauce.

Cut 12 rolls in half and place bottom halves in prepared pan.

Put 3 meatballs on each bottom roll and spoon remaining sauce over them.

Top with 8 oz torn mozzarella, 2 Tbsp grated Parmesan, and 12-24 basil leaves.

Place top rolls on and press down lightly.

Whisk 1 1/2 Tbsp melted butter with 3/4 tsp minced garlic, 3/4 tsp garlic powder, and 2 tsp minced parsley.

Brush garlic butter over roll tops.

Cover with foil and bake 20-25 minutes, then uncover and bake 5 more minutes until golden and cheese is melted.