Bring a large pot of salted water to a boil. Boil the pasta for 1 minute less than the pasta states.
While the water is coming to a boil, dice the onion and mince the garlic.
Heat the butter in a large, deep skillet over medium heat. Once melted, add the onions, salt, and red pepper flakes. Cook for around 5 minutes, stirring occasionally, to soften the onions.
Stir in the garlic and cook until fragrant, around 30 seconds.
Add the tomato paste to the skillet and stir to coat the onions and garlic. Cook for 2-3 minutes.
Pour in the vodka and heavy cream and stir well to combine. Let the sauce simmer over very low heat as the pasta finishes cooking.
Taste the sauce and add additional red pepper flakes if you’d like it more spicy.
Use a slotted spoon or strainer to scoop the pasta from the water directly into the sauce. Add a ¼ cup or so of pasta water to the sauce and stir it all together to combine.
Finish cooking the pasta for 1 minute in the sauce, adding additional pasta water as needed to thin it out a bit.
Use a microplane grater or the smallest holes on a cheese grater to grate Parmesan cheese over the top, stir to combine, and serve.