1 red onion, halved lengthwise and very thinly sliced
1 lemon, very thinly sliced, seeds removed
1 clove garlic, sliced
Kosher salt and freshly ground black pepper
One 1 1/2-pound piece center-cut skinless salmon fillet
4 tablespoons (1/2 stick) unsalted butter, melted
1 tablespoon dried Italian seasoning