In a medium bowl, whisk together egg yolks, milk, 1/2 cup cheese, lemon zest, and black pepper. Set aside.
In a 4-quart Dutch oven, cook linguine in a large amount of boiling salted water according to package directions, about 11 minutes. Drain, reserving 1/4 cup of the cooking water. Return linguine to Dutch oven.
Meanwhile, in a large skillet, cook sausage over medium heat for 5 minutes or until browned, adding garlic in the last minute of cooking.
Pour egg yolk mixture over linguine in Dutch oven. Add sausage and spinach; toss to coat. The heat from the pan and the linguine cooks and thickens the egg yolk, making a silky sauce. If needed, stir in enough reserved pasta cooking water to reach desired consistency. Toss with parsley.
Serve immediately. Sprinkle with additional Parmesan cheese, if desired.
