If the mussels have any stringy bits (called the beard) hanging from the shells, grab on to them with a kitchen towel [1] and pull down until they detach [2]. Using a scrub brush, clean the mussels [3].
Bring a large pot of water to a boil for the pasta.
In a small skillet, melt the butter over medium-high. Add 1 clove garlic; swirl until aromatic, about a minute. Add the breadcrumbs and stir often until deep golden brown, about 6 minutes. Transfer to a plate; let cool. Add the parsley.
In a large, deep skillet with a lid, heat the oil, two turns of the pan, over medium-high. Add the sausage; cook, stirring often and breaking up with a spoon, until browned, about 7 minutes. Add the onion, pepper, fennel and remaining garlic. Cover partially and cook, stirring often, until the vegetables soften, 7 to 8 minutes. Stir in the vermouth, then the stock. Reduce the heat to low and let the sauce simmer while the pasta cooks.
Salt the boiling water, add the pasta and cook until almost al dente.
Meanwhile, add the mussels to the sauce. Cover, increase the heat to high and cook, shaking the pan often, until the mussels open (discard any that don't open), about 3 minutes.
Drain the pasta. Toss the pasta with the sauce. Divide among shallow bowls; drizzle with EVOO and top with the breadcrumbs. (Don't forget to put out bowls to collect shells.)
