Preheat the oven to 450*F.
Mix the mayonnaise, rosemary, garlic and mustard together in a small bowl with a pinch of salt and pepper. Set aside.
Heat a large, oven-proof skillet over medium-high. Rub the flesh of the salmon with the olive oil and season generously with salt and pepper. Place the salmon, flesh side down into the hot skillet. Season the skin side. Let the flesh get a deep sear, about 3 minutes.
Carefully flip the salmon over. Turn off the flame. Spread the mayo mix over the tops of the salmon fillets, and into the sides. Sprinkle on the hazelnuts and pat into place. Put the skillet into the oven and roast until cooked thru, about 10 minutes for thick fillets.