In a large pot, fill with water, add a generous pinch of salt, and bring to a rolling boil. Add the linguine and cook according to package directions until al dente. Reserve 1/2 cup of the starchy pasta water before draining.

In a medium bowl, combine the ground beef, breadcrumbs, egg, 1/4 cup Parmesan, one third of the minced garlic, parsley, salt, pepper, and fennel seeds. Mix gently with your hands until just combined. Roll into 1-inch balls.

In a large skillet over medium-high heat, add 1 tablespoon olive oil and 1 tablespoon butter. Once the butter foams, add the meatballs in a single layer. Cook for 3 to 4 minutes per side, until deeply browned. Transfer to a plate and set aside.

Reduce heat to medium. Add the remaining 2 tablespoons butter and 2 tablespoons olive oil to the skillet. Once melted, add the remaining minced garlic and cook for 30 seconds until fragrant. Pour in the heavy cream and milk, whisking to scrape up any browned bits from the bottom.

Slowly whisk in the grated Parmigiano Reggiano cheese until the sauce is smooth and thick. Season with a pinch of nutmeg and black pepper. Taste and adjust salt.

Return the meatballs to the skillet along with any juices from the plate. Toss in the cooked pasta. Pour in the reserved pasta water, a little at a time, stirring until the sauce coats the pasta beautifully.

Sprinkle with extra Parmesan and fresh parsley. Serve immediately.