The stock: First of all, take the seabass and make sure that the gills have been thoroughly removed; wash the insides to get rid of all the blood (the taste of gills/blood will ruin the recipe). Fillet the fish with a sharp knife to obtain two fillets and skin them; chop the fillets into cubes and store away. Put all the fish trimmings (head, fishbones/spine, skins) in a pan together with the half onion (chopped into quarters; you can leave the skin on), two cloves of garlic (skin on, lightly crushed), the parsley stems and a generous lug of oil. Turn the heat on to medium and let the ingredients fry a little, stirring them from time to time to prevent burning. When the fish meat is flaking off the bones and the ingredients are fragrant (it will take ~8-10 minutes) add about 1 liter of water to the pan (no need to add it all, just enough that the ingredients are fully submerged) and let it warm up. When bubbles start to appear, signaling imminent boiling, lower the heat to low and let it simmer for 1 hour. After this time has passed, turn off the heat and filter, retaining the cooking liquor. This will be the fish stock we will use to cook the pasta. Add a pinch of sea salt to it and keep it warm.
The sauce: While the fish stock is simmering, wash the mushrooms gently with a wet cloth and slice them (~3 mm thick). Add a little olive oil to a frying pan, turn the heat on to medium, add the remaining clove of garlic (skin on, lightly crushed as before); when bubbles appear around the edges of the garlic clove, add the sliced mushrooms and a sprig of basil. Do not add salt at this stage, or the mushrooms will release their water. Let the mushrooms cook in the flavoured oil, stirring often and occasionally adding a little fish stock to prevent from sticking. You can also deglaze the pan with a little white wine if you have some available. Wash the cherry tomatoes in the meantime, halve them and add them to the pan with the mushrooms once these are almost fully cooked but have a nice bite to them. Crush the tomatoes with the back of a ladle and let them cook until they make a nice tomato sauce (it will take ~10-15 minutes). Add more fish stock along the way if it dries up and adjust the salt at the end (a couple of pinches of sea salt, approximately, but taste beforehand).
The pasta: Bring a pot of water to a vigorous boil, add a fistful of sea salt and drop the paccheri in it. The cooking time to "al dente" is 13 minutes usually, but we will remove them at ~10 minutes to continue with the stock. After these 10 minutes have passed, transfer the paccheri to the pan with the mushrooms and tomatoes, add the cubed fish fillets and a generous ladle of warm fish stock. Turn the heat on to high under the pan (as high as it goes) and let the pasta cook in the stock, replenishing one ladle at a time as it dries up on the bottom of the pan. Stir the pasta and toss it in the air several times - this will have the effect of breaking up the seabass cubes (creating a "ragù", as the fragments blend in with the sauce) and making the sauce creamier as the pasta releases its starch. This is the reason why it is important to use quality pasta in this recipe - trash pasta does not release starch and the sauce will therefore not come together. Continue the procedure until the sauce is thick and has come together nicely, and the paccheri are cooked just right: to me this means that if you cut one in half, you will see a very faint white line of undercooked pasta in the middle; you can also just taste them and cook to your liking, but if you overcook them they will break and come apart. It takes ~15 minutes to finish cooking the paccheri in the stock, so altogether 10+15= ~25 minutes from the moment you added them to the pot of water.
Serve the paccheri with a light grating of pecorino cheese and a couple of basil leaves on top.