Melt 1 tablespoon of the butter in a large pot over medium-high heat. Add the onions and garlic and cook, stirring occasionally, until the onions begin to soften and become translucent, about 4 minutes. Add the tomatoes, cayenne and 1/2 teaspoon salt and cook for 2 minutes. Add the wine and bring to a simmer.
Add the mussels, cover the pot and cook until they open, 3 to 5 minutes. (Check halfway through and transfer any open mussels to a serving bowl.) Add the remaining tablespoon of butter, swirl the pot a few times to incorporate the butter into the broth and then toss the mussels with the broth. Transfer the mussels to a serving bowl, discarding any that did not open. Pour the broth over the mussels, sprinkle with the parsley and squeeze the lemon over top. Serve with Grilled Garlic Bread.
Grilled Garlic Bread:
Heat a cast-iron grill pan over medium-high heat. Meanwhile, stir together the butter, Italian seasoning, garlic powder and 1 teaspoon salt in a small bowl. Spread the mixture on both sides of the bread. Grill until toasted and grill marks appear, about 2 minutes per side.