Heat oil in a large heavy pot over medium heat. Cook onion, stirring occasionally, until golden and softened, for about 8 to 10 minutes. Add the garlic and red pepper flakes (if using) and season with some salt. Cook, stirring often, until fragrant and garlic is softened, for about 2 minutes.
Add the tomato paste and cook, stirring occasionally, until slightly darkened in color, for about 3 to 4 minutes. Pour in the wine and cook, stirring often, until the smell of the alcohol is almost completely gone, about 4 minutes. Add the crushed tomatoes and increase the heat to medium high. Cook, stirring often, until the sauce thickens slightly, for about 8 to 10 minutes. Taste and season sauce with salt.
Meanwhile, cook pasta in a large pot of boiling salted water, stirring occasionally, until al dente. Reserve 1 cup of the pasta cooking liquid and drain the rest.
Back to the sauce: Add in the mussels, shrimp, and ¼ cup pasta cooking liquid to the sauce. Cover and let cook until the mussels open, for about 3 to 4 minutes. Feel free to flip the shrimp, halfway through cooking. Using tongs, pick out the shrimp and mussels and transfer to a large bowl, discarding any mussels that have not opened.
Add in the cooked pasta and another ¼ cup pasta cooking liquid to the sauce and combine with the sauce. Reduce the heat to medium, add the butter, and continue to cook, stirring and adding more pasta cooking liquid if needed, until the sauce coats pasta, about 2 to 3 minutes. Remove from the heat, add the shrimp and mussels back to the pot, and toss to combine. Garnish with parsley and serve.