Position a rack in the center of the oven and heat the oven to 400°F. Line a heavy-duty rimmed baking sheet with foil.

In a small bowl, stir together the mayonnaise, mustard, lime juice, and tarragon.

Arrange the salmon skin side down on the baking sheet and sprinkle lightly with salt. Spread the mayonnaise mixture evenly over each fillet (there may be a little left over).

Roast the salmon until just cooked through, 10 to 14 minutes.

(To test for doneness, poke a paring knife all the way through the thickest part of one fillet and hold it there for 5 seconds. Then touch the flat side of the knife gently to your lower lip. If the knife feels warm, the fish is cooked through.) Using a spatula, lift the fillets off the baking sheet, leaving the skin behind, and transfer to warm plates.