For the meatballs. Put the breadcrumbs in a large bowl and add the milk, ground lamb, oregano, garlic, egg, shallot and some salt and pepper. Gently mix until everything is combined, then form into golf ball-size spheres. Take a cube of the feta and push it into a meatball, wrapping the meat mixture around the cheese to completely encase it. Continue with the remaining meatballs and feta cubes. Refrigerate the meatballs for 30 minutes if you have time.
When you are ready to cook the meatballs, heat a large skillet over medium-high heat. Add 2 tablespoons of oil. When the oil is hot, add the meatballs and brown them on all sides, 5 to 6 minutes. Remove them from the skillet and set aside.
For the sauce. Reduce the heat under the skillet to medium, then add the onion and cook, stirring and adding an additional tablespoon or two of oil if needed, until softened slightly, 1 to 2 minutes. Add the marinara, red pepper flakes if using and olives and return the meatballs to the skillet. Cook for another 10 minutes to make sure the meatballs are cooked.
For serving: Meanwhile, cook the pasta in salted boiling water according to the package directions, then drain.
Serve the meatballs and sauce over the pasta, then garnish with crumbled feta and chopped parsley.