In a large bowl, use your hands to mix the ground beef, sour cream, egg, parmesan cheese, kosher salt, and black pepper. Wet your hands and shape the mixture into 12 meatballs, each weighing about 2 ounces.
In a large (12-inch) lidded saucepan or skillet, heat the olive oil over medium heat. Add the garlic and cook, stirring, about 1 minute, just until golden. Add the tomato sauce, kosher salt, black pepper, onion powder, oregano, and cayenne. Bring to a boil.
Add the meatballs, spooning the sauce on top. Cover, reduce the heat to medium-low, and simmer the meatballs until they are cooked through, about 30 minutes. Start checking on them after 20 minutes - depending on your stove, they might be ready faster.
Remove the lid and turn the cooked meatballs in the sauce to coat. Garnish them with parsley and serve.
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Notes
If using any salt other than Diamond Crystal Kosher Salt, including Morton kosher salt, use half the amount listed.
For tender and airy meatballs, handle the beef mixture lightly. Try not to overwork it, or the meatballs will turn out dense.
Please use 85/15 or 80/20 ground beef. Leaner than that would result in dry, crumbly meatballs that fall apart. The fat is needed not just for flavor but also to hold the mixture together.
The sauce is mildly spicy. Depending on your personal preferences, you might consider using just ⅛ teaspoon of cayenne pepper. You can also omit the cayenne.
You can keep the leftovers in an airtight container in the fridge for 3-4 days. Reheat them covered in the microwave at 50% power. You can also freeze the cooled leftovers for up to three months. Thaw them overnight in the fridge before reheating.