Heat oven to 375° F. In a deep 8- to 10-inch ovenproof skillet, heat the oil over moderately high heat. Add the sweet potatoes and onion and cook, stirring, until tender, about 5 minutes. Cook 5 minutes for the salmon or until cooked through. Remove the skillet from heat.
Whisk together the eggs, milk, cheese, tarragon, salt, and a few grinds of pepper and add to the skillet. Stir. Bake in the upper third of the oven for 20 minutes or until set. Transfer the frittata to a plate; cut into wedges.