Combine the black pepper, paprika, cayenne pepper, minced garlic, Dijon mustard, brown sugar, onion powder, and salt in a small bowl. Stir in 1 tablespoon of olive oil to make a paste. Spread the paste all over the salmon fillets, and set aside to marinate at room temperature 30 minutes.
Heat the remaining 2 tablespoons of olive oil in a small pan over medium heat. Stir in the onion, and cook until tender and golden brown, about 10 minutes. Heat a separate non-stick skillet over medium-high heat. Cook the salmon fillets in the hot skillet until golden brown on each side, and no longer translucent in the center, about 4 minutes per side. Pour the browned onions and olive oil over the salmon fillets to serve.