Grate the bread on a box grater or pulse it into crumbs in a food processor. Toss the breadcrumbs with the milk in a small bowl and let sit 5 minutes to rehydrate.
Combine the breadcrumbs, beef, Parmesan, parsley, onion, egg, garlic and salt in a large bowl and mix until combined.
Using your hands, gently form the meat mixture into 18 balls slightly larger than golf balls. (Don't pack the meat together too tightly or the meatballs will be tough.) Refrigerate at least 1 hour and up to 1 day.
Preheat the oven to 375 degrees F. Bake the meatballs on a baking sheet until cooked through and tender, 20 to 25 minutes.
Serve immediately with the Quick Marinara Sauce and spaghetti or on sandwiches. The baked meatballs can be refrigerated up to 3 days or frozen in an airtight container up to 6 weeks.
Quick Marinara Sauce:
Heat the oil in a medium saucepan over medium-high heat. Add the onion and garlic and cook, stirring, until lightly browned, about 5 minutes. Add the tomatoes, thyme and basil and bring to a boil. Reduce the heat and simmer, uncovered, until thickened, about 15 minutes.
Remove and discard the thyme stems. Stir in 2 teaspoons salt and 1/2 teaspoon pepper. The sauce can be refrigerated up to 3 days or frozen in an airtight container up to 2 months.
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