Season shrimp with ½ teaspoon of the salt and ¼ teaspoon of the pepper. Heat 1 tablespoon of the butter over medium-high heat in a large pot big enough to cook the pasta. Add the shrimp and cook, stirring occasionally until opaque and pink, 3-4 minutes. Transfer it to a plate.
Add 1 tablespoon of the butter to the pot. Add the onion. Cook until softened, stirring occasionally.
Remove the pot from the heat and add the garlic, red pepper flakes, and lemon zest. Stir for a minute.
Add the remaining 1 teaspoon of the salt, ¼ teaspoon pepper, stock, white wine, and heavy cream.
Return the pot to the stove. Bring to a simmer over high heat, stirring frequently. Once it reaches a simmer, immediately stir in the pasta.
Reduce the temperature to keep it at a low simmer, stirring often, until the pasta is cooked, 13-15 minutes. If the pasta has soaked up a lot of the liquid and seems dry, add an extra ½ cup of stock.
Once the pasta is cooked, stir in the cooked shrimp. Heat through. Stir in remaining 2 tablespoons of butter and then add fresh lemon juice.
Top with chopped fresh parsley, if using.
