Heat olive oil in a skillet over medium-high heat.
Add garlic and cook for 1 minute, or until fragrant, stirring frequently.
Whisk in flour; whisk until flour isn't dry.
Slowly whisk in the broth.
Cook and whisk until mixture is smooth; continue to cook until mixture boils and thickens. Remove from heat.
Whisk in parmesan cheese; whisk until melted.
Whisk in the milk; whisk until combined.
Season with salt and pepper.
Stir in lemon zest and lemon juice.
Sauce will continue to thicken as it stands. If sauce is too thick, add a little more milk.
Drain pasta and return to pot.
Pour sauce over the cooked pasta and shrimp; mix until combined.
Stir in 1 tablespoon parsley.
Transfer to plates; garnish with remaining parsley and a sprinkle of shredded parmesan cheese.