Preheat the oven to 450°F.
Set a large ovenproof pan over medium heat and add the grapeseed oil. Season the salmon with salt and pepper on both sides and place skin-side down in the pan. Cook until the skin gets golden around the edges, 3 to 4 minutes. Transfer the pan to the oven and bake for 7 to 9 minutes, or until it reaches an internal temperature of 135°F.
Meanwhile, in a small bowl, whisk together the fish sauce, tomato paste, vinegar, brown sugar, mustard seeds, cayenne, and ¼ cup water.
Set a medium pan over medium heat and add the butter. Let it melt and begin to brown slightly, 2 to 3 minutes. Add the onion and cook until softened, 3 to 4 minutes. Pour in the fish sauce mixture. Turn off the heat and stir to combine, seasoning with a generous pinch of salt.
When the salmon is out of the oven, gently flip it into the pan with the sauce, exposing the crispy skin side. Transfer to a plate, taking a scoop of sauce for each fillet, and pooling any additional sauce around and on top of the salmon.