Heat oven to 375º 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.
Add the salmon. Cook, stirring, for 5 minutes or until cooked through. Remove the skillet from heat.
Whisk together the eggs, milk, cheese, tarragon, salt, and a few grinds of pepper. Add to the skillet. Stir.
Bake the frittata in the upper third of the oven for 20 minutes or until set. Transfer to a plate. Cut into wedges.