Put the ricotta in a cheesecloth-lined sieve set over a bowl; let drain in the refrigerator overnight.
Preheat a large skillet over medium heat. Add the olive oil, diced fennel, onion and garlic. Reduce the heat to medium low and cook until the vegetables are soft, about 10 minutes; let cool completely.
Meanwhile, put the bread in a bowl, add the milk and soak about 5 minutes. Mix the beef, veal and pork in a large bowl. Add the onion mixture, egg, lemon zest, red pepper flakes, fennel seeds, pecorino, parmesan and parsley. Drain the bread, add it to the meat mixture and mix until combined. Season with salt and pepper.
Mix the ricotta and salt to taste in a bowl; transfer to a pastry bag with a round tip. Form the meat mixture into 24 meatballs (slightly larger than golf balls). Make a 1-inch indentation in each meatball with your thumb, then pipe in about 1 teaspoon ricotta and form the meat around it to cover the cheese.
Heat the canola oil in a large skillet over medium heat. Add the meatballs and cook until golden brown on all sides, about 7 minutes; set aside. Warm the tomato sauce in a large saucepan over medium heat. Put the meatballs in the tomato sauce and simmer until cooked through, about 15 minutes.
Divide the meatballs and sauce among bowls; serve over bucatini, if desired. Drizzle with olive oil. Sprinkle with a pinch each of red pepper flakes and fennel pollen, if desired.
Photograph by Joseph DeLeo
Recipe courtesy Missy Robbins for Food Network Magazine