Cooking mussels in a slow cooker? You bet! It's the right tool for building the rich and spicy broth in which these shellfish cook to perfection. Use the freshest mussels you can find, and have the fishmonger clean them for you.
Heat the olive oil in a large nonstick skillet over medium-high. Add the sliced fennel, onion, carrot, and celery, and cook, stirring occasionally, until starting to brown, about 8 minutes. Add the garlic, and cook, stirring occasionally, until fragrant, about 2 minutes. Add the white wine; reduce the heat to medium, and cook until the wine is mostly reduced, 4 to 5 minutes. Transfer the mixture to a 6-quart slow cooker; stir in the tomatoes, bay leaves, lemon halves, salt, and red pepper. Cover and cook on HIGH until the vegetables are very tender, about 3 hours.
Add the mussels to the slow cooker; cover and cook on HIGH until the mussels open, 30 to 40 minutes. Discard the bay leaves, lemon halves, and any mussels that do not open.
Meanwhile, cook the linguine according to the package directions, omitting the salt and fat. Divide the pasta, mussels, and sauce among 4 shallow bowls. Garnish with the fennel fronds, if desired.