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. Add the fish or meat; cook, stirring, for 5 minutes for the salmon, 10 minutes for the chicken, or 15 minutes for the pork, 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.