Prepare the spicy marinara sauce. Heat 1 tablespoon oil in a large pot over medium-high heat. Add the chopped onions and saute until softened, about 3-4 minutes. Add the garlic and saute 1 more minute. Add the remaining ingredients (canned tomato – red pepper flakes) and bring to a boil. Reduce heat to low and simmer.
Make the meatballs. Combine the white bread with buttermilk and let sit for 5 minutes. Squeeze out the excess liquid and crumble the bread into small pieces. Combine bread with remaining ingredients (ground beef – black pepper), mixing until well combined. Form about 18 meatballs and carefully drop them into the sauce. Continue to cook the meatballs in the sauce until they are firm, about 20 minutes.
Meanwhile, cook the broccolini. Heat 1 tablespoon olive in a medium saute pan over medium heat. Add the broccolini and sear on all sides, cooking until the thickest part of the stems are tender. Season with the juice of ½ a lemon, salt, and pepper. Set aside.
Assemble the sandwiches. Cut the submarine bread in half and stuff with broccoli rabe, 3 meatballs, and burrata. Finish in the broiler if desired for 1-2 minutes or until the tops of the sandwiches are slightly charred. Serve immediately.