Step 1To cut the salmon in escalopes: Set the fillet on a cutting board, tail end facing away from you. With a long, sharp knife, slice the fillet crosswise on the bias toward the tail end, discarding the skin when you reach it. Slices should be wide and flat -- about 4 to 5 inches wide and about three-eighths-inch thick. Set them aside on a plate.
Step 2Bring a pan of salted water to a boil, add the leek and carrot and blanch, about 2 minutes. Drain, rinse with cold water and drain them again thoroughly. Set aside four tarragon sprigs for decoration; discard the remaining stems and chop the leaves.
Step 3Sprinkle the salmon escalopes each with a pinch of salt and a small pinch of pepper. Heat a cast iron or nonstick frying pan over high heat until very hot, then brush it with butter and add as many escalopes as will fit in a single layer. Cook until lightly browned on one side, about 1 minute. Turn and brown the other side, about 1 minute longer. Transfer them to another plate. Cook the remaining escalopes in the same way, cover with foil and keep them warm.
Step 4Add the cream to the pan and bring it to a boil. Whisk in the mustard, add the vegetables and heat gently for 1 minute, swirling the pan to mix in the vegetables. Swirl in the tarragon. Taste and adjust seasoning of the sauce with salt, pepper and more mustard.
Step 5Arrange the cooked escalopes on individual warm plates and spoon enough sauce over them to partially coat the salmon, leaving the pink edges. Top with a sprig of tarragon and serve.