Position oven rack 4-6 inches from the heating element, and preheat the broiler.
Season salmon all over with salt and pepper. In a small bowl stir together the Thai sweet chili sauce, Sambal Oelek, and mustard.
Pour the mixture into a large cast-iron skillet. Set skillet over medium-high heat until the mixture boils.
Lay the salmon skin side up in the sauce. Cook the salmon until it turns pale pink about 2 minutes, then move the skillet to the oven and broil until the skin gets crispy (about 3-4 minutes). Don’t burn the sauce!
Add 1/4 cup water to the skillet and cook over high heat, stirring and scraping until the sauce thickens. Pour the sauce from the skillet onto the salmon. Serve immediately.