In a large bowl, mix together all of the meatball ingredients until fully combined. I find the meatballs have the best texture if you use a handheld food chopper or food processor to mince the onion and garlic as finely as possible. You can knead the mixture with your hands until just combined, or mix it in a stand mixture with a paddle attachment.
Form balls with the meat, one heaping tablespoon at a time. Drop them onto a cookie sheet as you form them. You should be able to make about 24 balls. If your meatballs seem too soft to keep their shape in a pan, place your cookie sheet in the freezer for 5 minutes.
Heat nonstick spray in a large skillet over medium-high heat. Brown the meatballs on all sides until they are cooked through, about 10 minutes. Remove meatballs from the saucepan and set aside. Make sure you save all of those delicious browned bits sticking to the bottom!
Melt the butter in the skillet. Whisk in the flour and cook for a minute. Pour in the broth, whisking constantly, until everything is incorporated. Simmer until thickened and glossy, about 10 minutes. Add in the milk, mustard, pepper flakes, salt, and pepper. Simmer until thick and creamy, about another 5 minutes.
Place the meatballs in the sauce and cook until they are heated through, just a few minutes. Serve over pasta or with mashed potatoes.