In a large bowl, mix the chopped mortadella with ground beef, fennel seed (if using), salt, garlic, parmesan, and black pepper.
Add panko bread crumbs and the egg, mixing until the mixture is well combined and tacky.
Use a cookie scoop to form the mixture into approximately 20 meatballs. Place them on a sheet tray and roll into balls.
Heat a large sauté pan over medium-high heat. Add enough olive oil to cover the bottom. Once hot, add the meatballs in a single layer.
Sear the meatballs for about 2 minutes on one side, then turn to sear additional sides until golden brown. (They don’t need to be fully cooked at this stage.) Remove and set aside.
In the same pan, reduce the heat to medium. Add the garlic and sauté for about 5 minutes.
Add red pepper flakes, sauté for 30 seconds, then add the crushed tomatoes. Stir and season with salt and a pinch of sugar.
Return the meatballs to the pan, bring to a simmer, then reduce heat to medium-low. Simmer for 5-8 minutes, adding basil halfway through, until the meatballs are cooked through.
Bring a large pot of salted water to a boil. Add boxed spaghetti and cook according to package instructions until al dente.
Using tongs, transfer the spaghetti directly into the pan with the sauce, allowing a bit of pasta water to cling to the noodles. Toss until the pasta is evenly coated in the sauce.
Serve the spaghetti in shallow bowls, adding some reserved sauce on top, along with 2-3 meatballs.
Finish with freshly grated pecorino or parmesan and a sprinkle of chiffonade basil.
