Prepare the spicy marinara sauce. Heat 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 for 1 more minute. Add the remaining ingredients for the marinara sauce including canned crushed tomatoes, white wine, dried parsley, oregano, basil, and red pepper flakes. Bring to a boil, reduce heat to low, and simmer for 10 minutes. Adjust seasoning with salt and pepper.
Make the meatballs. Combine the white bread with milk and let sit for 5 minutes. Squeeze out the excess liquid and crumble the bread into small pieces. Combine bread with remaining ingredients for meatballs including ground beef, ground pork, grated parmesan, egg, garlic, Italian seasoning, salt, and pepper, mixing until well combined. Form about 15 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 of olive in a medium saute pan over medium heat. Add the broccolini and sear on all sides. Add water and cover the pan. Cook until all of the water has evaporated and 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 broccolini, 3 meatballs, and 2 ounces burrata. Spoon extra sauce on top and serve immediately.
Notes
The meatballs with the spicy marinara and broccolini can be prepared ahead of time. Right before serving, heat up the meatballs and broccolini and assemble the sandwiches.