Heat oil in a large wide pot over high. Cook onion, stirring occasionally, until edges are blackened, 6–8 minutes. Let pot cool slightly, then add butter, garlic, arranging cut side down, and thyme and cook over low heat until garlic is golden and fragrant, about 3 minutes. Add kombu, bay leaf, and stout and increase heat to medium-high. Add shellfish; cover pot. Cook, shaking pot often and transferring shellfish to a large bowl as they open, until all the shellfish are open, 5–8 minutes. Discard any that don’t open. Season cooking broth with salt and pepper. Divide shellfish among bowls; ladle broth over.