1. Bring a large pot of salted water to a boil. Cook pasta until al dente (each pasta will cook differently depending on the brand, so use package instructions). Drain, reserving about 1 cup of pasta water. Return pasta to the pot, drizzling with a little olive oil to prevent pasta from clumping together while you finish the sauce.
2. Meanwhile, heat ¼ cup oil a large skillet over medium heat. Add fennel, season with salt and pepper, and cook, stirring occasionally, until nicely caramelized, about 5-8 minutes. Add fennel seed and crushed red pepper flakes, stirring a minute or two to toast in the oil. Add wine and let simmer until it’s reduced by about half and the fennel is softened.
3. Add 4 cloves of your well-crushed garlic cloves, and cook a minute or two, then add your tomatoes maybe squeezing them a bit as you add (it’s fun). Season with salt and pepper and simmer until the tomatoes turn juicy and jammy but are still chunky, 5-8 minutes. Add 1 cup of water and simmer for 5 or so minutes, until everything has melded together and the brothy sauce has thickened slightly, seasoning with salt, pepper and crushed red pepper flakes as needed.
4. Add shrimp and mussels to the sauce, cover the pan (use a sheet pan or large plate, whatever works), and cook, stirring occasionally, until the shrimp are bright pink and opaque and the mussels have opened, 5–8 minutes.
5. Add the sauce to the pasta in the larger pot (or the pasta to the sauce–whichever vessel holds more), gently tossing to evenly coat the pasta with the briny sauce and distribute the seafood (tongs are best for this). Add chopped fennel fronds, remaining 2 cloves finely chopped garlic and toss, toss, toss.
6. To serve, transfer pasta to a large bowl or platter (or serve straight from the pot), finishing with a squeeze of lemon, a pinch of flaky salt, crack of pepper, and more crushed red pepper flakes. Scatter basil (and more fennel fronds if you have them) over top. Eat with some crisp white wine and sparkling water, and don’t forget to bring a bowl for shells to the table.