Cook the Pasta - Bring a large pot of salted water (1 Tbsp salt to 1 gallon water) to a boil. Cook the pasta to 1 minute before al dente, so you can finish it in the sauce. Reserve 1 C of pasta water and drain the rest.
Make the Sauce - While the water boils and pasta cooks, melt the olive oil and half the butter over medium heat in a large skillet or pan (large enough to hold the pasta and sauce later).
Add the garlic, (optional) red pepper flakes, and a heavy pinch of salt. Sauté for a minute.
Add the tomatoes and cook 3-5 minutes until softened.
Deglaze with the white wine and bring to a simmer. Let the sauce reduce for 2-3 minutes.
Add the drained mussel meat to the sauce and cook per below:Frozen, pre-cooked - 4-5 minutesFrozen, raw - 6-8 minutesFresh, pre-cooked - 2-3 minutesFresh, raw - 4-6 minutesThe safe cooked temp for mussel meat is 145 degrees F. The meat should be opaque and firm, but still tender. Bring it All Together - Toss the pasta with the sauce and mussels. I like to use tongs here and mix the pasta pretty roughly with the sauce. The sauce will be watery to start, but will thicken as it mixes with the starch in the pasta. This should take roughly 5 minutes, but mix as long as you want.
If necessary, add the reserved pasta water a little at a time to loosen up the sauce. You don't need to add all of it, and I usually end up using 3/4 C. Be sure to mix well to help form an emulsification between the sauce and pasta water. Cut the heat, add the parsley and remaining butter, and squeeze in the juice from half the lemon. Mix to melt the butter and combine all the ingredients.
Season with salt and pepper to taste.
Serve hot with the other half of the lemon (cut into wedges), some crusty garlic bread and maybe a glass of white wine.