Put a large pot of water over high heat to bring to a boil.
Mix the sherry and saffron together in a measuring cup and set aside.
In a large pan that will eventually fit everything—I use a braiser pan or Dutch oven—heat the pancetta over medium-low heat with the smashed garlic clove for about 10 minutes, until it crisps and renders all of its fat.
While this is happening, dissolve the baking soda in a large bowl of water and plunge the mussels into the water. Let them soak for about 5 – 10 minutes, ensuring they spit out any sandy grit they’re holding on to.
Add the fennel and chili flakes to the pan and stir to bloom their flavors for just about 30 seconds. Turn the heat up a little before pouring in the golden sunburst-tinted sherry. Deglaze any porky bits stuck to the pan and then add in the crushed tomatoes. Bring this to a gentle boil.
At this point, the water should be boiling. Salt it heavily, add the pasta, and stir to incorporate, boiling per package instructions for al dente (usually about 8 – 9 minutes). Tumble the mussels into the pancetta-tomato pan, cover and reduce the heat to medium-low while the pasta boils.
After about 8 – 9 minutes not only should the pasta be just about al dente, but the mussels should be just about opened. Drain the pasta, reserving a little water just in case, before dumping it into the pan with the mussels. Toss everything to combine, adding a bit of pasta water as need to help coat the spaghetti. The mussels will finish opening in the residual heat of the pan (although, any that haven’t opened at all should be discarded).
Scatter the dish wit parsley and transfer to two warmed shallow bowls or rimmed plates and devour with some rustic bread.
To clean the mussels, scrub them with a stiff brush under gentle, cold running water to remove any outside grit. Any mussels with beards—those little bristly hairs that stick out from inside the shell—need to be cleaned. Simply grab ahold of the beard and pull back towards the “hinge” of the mussel to remove.
Smaller mussels are preferred, as they are more likely to stay tender and less so to become rubbery. If yours are very small you may want to plan on only 5 - 6 minutes steaming to be safe. No matter the size they don't need to be fully opened before the pasta goes in the pan--just about halfway, more or less--so the pasta has time to soak up some sauce and the mussels don't overcook.
For any mussels that are slightly open before soaking, tap on them with the spine of a knife or against the counter (not to forcefully) or try to press them closed gently. If they don't clamp shut, throw them away--they're dead. The same goes for any mussels that don't open after the steaming is done--don't try to pry them, just pitch 'em.