Heat the oil in a large skillet over medium heat. Add the onion, carrot, celery and garlic and cook 4 to 5 minutes. Stir in the flour and tarragon until well combined. While stirring, slowly mix in the stock. Bring to a simmer and cook until the vegetables are tender, about 10 minutes. Add the salmon and simmer 4 to 5 minutes more, or until the salmon is just cooked through. Mix in the peas and then remove from the heat. Boil the potatoes in lightly salted water until tender. Drain well, thoroughly mash, and then whip in the buttermilk, butter, salt and white pepper.
Preheat the oven to 425 degrees F. Divide the salmon mixture among four small baking dishes; top with the mashed potatoes. Bake for 15 to 20 minutes, or until the salmon mixture is bubbly and the potatoes are golden.