Make the mushroom marinara by heating the olive oil in a Dutch oven over medium heat. Add the onions and mushrooms and cook stirring often, until tender, about 5 minutes.
Add the garlic, basil, oregano, and crushed fennel seeds, and cook for 60 seconds, stirring frequently.
Add the remaining ingredients. Simmer, stirring occasionally, for at least 2 hours. Taste and re-season if needed.
Make the meatballs by placing the mushrooms, onion, and garlic in a food processor to chop, otherwise, chop them as small as you can.
Combine all ingredients (except for olive oil) together gently. Form into small meatballs and set aside.
Cook the meatballs by heating a large skillet over medium heat with 1 tbsp olive oil. Cook the meatballs until golden brown on all sides, about 5-7 minutes.
Remove from the skillet and place the cooked meatballs in the mushroom marinara to simmer for 15 minutes.
To assemble the meatball sandwich, place meatballs with some sauce down the center of the the hoagie rolls. Sprinkle the top with mozzarella and Parmesan.
Place on a baking sheet, under the broiler and melt the cheese–watch carefully so don't burn.