1. Position oven rack in center of oven and preheat to 400°F.
2. Rinse and pat dry salmon then lay fillets about 2 inches apart on a large baking tray, skin side down.
3. Season fillet tops with salt and pepper.
4. In a small saucepan, whisk marmalade and vinegar over medium heat just until melted.
5. Let cool slightly before spreading over the top of fillets. Next top with onions and divided pats of Plugrá Butter.
6. Bake until fillets are just cooked through and opaque, about 12 minutes.