Rinse your bass fillets under cold water, making sure all scales are off, then pat dry and leave skin side down on a dry paper towel.
Trim and peel your onions, then cut them in half. Heat a large sauté pan over medium heat and add 2 Tbs. olive oil, then add the onions and season with salt, cook the onions cut side down. When the onions begin to sizzle place the whole pan in the oven for 15 minutes. Remove the pan and flip the onions over in the pan add the thyme, then set aside as you cook the fish.
In a large pan or two medium-sized ones, heat the remaining olive oil (adding more if necessary as the skin should not stick with this technique).
Season the fish nicely on both sides with salt and place into pans of smoking oil, skin side down. Shake free from the pan as you add them, and if the skin sticks don’t mess with the fish for a minute or so.
Cook over high heat getting a nice sizzle on the skin for 2-3 minutes. Baste the fish with hot oil from the pan, squeeze lemon over them and turn the heat off as the fish should cook 90 percent of the time skin side down.
Flip over gently with a fish spatula and allow the fish to finish cooking for 15-30 seconds, then serve alongside roasted onions with crispy thyme as a garnish.