In a large mixing bowl, combine the ground beef or pork, egg, breadcrumbs, grated Parmesan, chopped parsley, minced garlic, salt, and black pepper. Gently mix the ingredients with your hands until just combined; be careful not to overmix to keep the meatballs tender. Once mixed, form the mixture into 1-inch meatballs, aiming for about 20 to 24 pieces.

Heat a tablespoon of olive oil in a large skillet over medium heat. Once the oil shimmers, carefully add the meatballs in a single layer, making sure not to overcrowd the pan. Sear the meatballs for 6 to 8 minutes, turning occasionally until they are golden brown and cooked through. Once done, transfer the meatballs to a plate and set aside.

In the same skillet, reduce the heat to low and add 4 tablespoons of unsalted butter. As it melts, add minced garlic and optional red pepper flakes, stirring for about 1 to 2 minutes until fragrant. Pour in fresh lemon juice and toss in the reserved meatballs, letting them simmer in the garlic butter for 5 to 7 minutes, ensuring they're evenly coated.

While the meatballs are simmering, fill a large pot with water, add a generous pinch of salt, and bring to a rolling boil. Cook the linguine according to the package instructions, usually around 8 to 10 minutes, until al dente. Before draining, reserve about 1 cup of pasta water to adjust the sauce's consistency later.

In a separate saucepan over medium heat, melt an additional tablespoon of butter, then pour in the heavy cream. Allow it to simmer gently, stirring occasionally, until it thickens slightly. Add in grated Parmesan cheese, mixing until melted and creamy. Use reserved pasta water to adjust the sauce to your desired consistency, and season with salt and pepper.

Once the linguine is cooked and drained, add it directly to the creamy sauce, tossing until well coated. Serve the linguine onto plates, topping each serving generously with the garlic butter meatballs. Finish with a drizzle of extra garlic butter from the skillet, and garnish with additional chopped parsley and Parmesan for a delightful presentation.