Pasta with vodka sauce is a decadent, crave-able classic with a subtle spicy kick. For this version, we added mussels, which give the sauce a delicious briny touch that pairs really well with the creamy tomato flavors.
Bring a large pot of salted water to a boil. Heat the olive oil in a Dutch oven over medium-high heat. Add the onion and cook, stirring, until softened, 3 to 5 minutes. Stir in the garlic and red pepper flakes and cook 30 seconds. Stir in the tomato paste and cook, stirring, 1 minute. Add the tomatoes and their juices and 1/2 teaspoon salt. Bring to a boil, then reduce the heat and simmer, stirring occasionally, until thickened, 12 to 15 minutes.
Add the heavy cream to the sauce and simmer until thickened, 2 to 3 minutes. Stir in the vodka. Add the mussels, cover and cook until the mussels open, about 5 minutes (discard any mussels that do not open). Remove from the heat; keep the pot covered.
Meanwhile, after adding the cream to the tomato sauce, add the spaghetti to the boiling water and cook as the label directs for al dente. Reserve 1 cup cooking water, then drain. Add the spaghetti to the pot with the mussels, along with 1/4 cup cooking water. Cook over medium heat, tossing and adding more cooking water, a few tablespoons at a time, if the sauce seems too thick, until the spaghetti is well coated. Season with salt.
Divide the spaghetti and mussels among shallow bowls. Sprinkle with the parsley and more red pepper flakes.