Season salmon fillets with kosher salt. Line a baking baking sheet with foil and brush lightly with vegetable oil.
Preheat oven's broiler on high and set the oven rack about 8 inches from the heat source.
Process garlic, tarragon, and parsley in a blender or mortar and pestle to form a loose paste.
Mix mayonnaise, Dijon mustard, lemon juice, and cayenne pepper into garlic paste until combined.
Place salmon fillets skin side down on the baking sheet. Spoon herb spread over the top and sides of each fillet.
Cook under the preheated broiler until fillets are well-browned, about 5 minutes. Turn the broiler off and turn the oven to 350 degrees F (175 degrees C).
Bake until the internal temperature of the salmon is 130 degrees F (55 degrees C) and salmon flakes easily with a fork, about 3 to 4 minutes.
