Make the meatballs and start them braising
Run the bread slices under running water to soak it. Wring as much water out if it as you can. Chop the wet bread as finely as you can and place it in a large bowl for mixing.
Add the 1/3 C Parmesan as well as the garlic, ricotta, prosciutto, parsley, and eggs.
Grind the fennel seeds in a spice grinder or crush them finely in a mortar and pestle. (Adding the 1-1/4 tsp salt to the fennel seeds can help it to grind better.)
Add the fennel, salt, pepper, oregano, nutmeg, red chili (if using), and pepper flakes to the chopped bread and seasonings.
Add the ground beef. Break the beef into small slumps and then mix the entire mixture thoroughly by hand. Mix it until it just becomes homogeneous and smooth. Don't overmix beyond that point.
Oil your hands with some olive oil and roll 1/4-cup portions of the meat into balls. Set them aside on a plate or a baking sheet.
Start reheating the pasta sauce in a deep skillet. If you made if fresh just now, then keep it hot.
Once all the meat has been rolled into balls, heat 1 Tbsp oil in a skillet over medium-high heat and brown the meatballs well on at least two sides. They will not be cooked through, and that is just fine.
If your pan is too small to cook them all in one batch, cook them in as many batches as needed, giving them space between each other for steam to escape. Transfer the browned ones to a plate or the sheet pan again.
Once all the meatballs are all browned, place them gently in the pan with the sauce and simmer them together. Simmer for about 10 minutes, then start preparing the pasta.
Prepare the pasta and finish the dish
While the meatballs simmer, bring a large pot of well-salted water to a boil.
Add the spaghetti to the water and use a timer, like the Extra Big & Loud Timer, to time it according to the package's instructions. Good-quality pasta will take about 10 minutes.
Using an instant-read thermometer like Thermapen ONE, check the temperature of the meatballs. They need to have a minimum temperature of 160°F (71°C). Once the meatballs reach the minimum temperature and the pasta timer is almost up, remove the meatballs to a separate platter and drain the pasta, reserving about a cup of the starchy pasta water. Do not rinse the noodles.
Add the cooked pasta to the sauce along with enough pasta water to loosen the sauce slightly. Cook the pasta together with the sauce over medium-high heat for about one minute, until the sauce starts to thicken again and clings nicely to the noodles.
Plate the pasta for serving and either top with the meatballs or serve them on a plate alongside. Garnish with shredded parm and freshly torn basil leaves.