Heat the oil in a large skillet over medium heat. Sprinkle the chicken, pork, or fish with the salt and pepper. Sauté until cooked through, 8 to 10 minutes per side for the chicken, 12 to 15 minutes per side for the pork, or 5 to 7 minutes per side for the fish. Remove from the skillet, cover, and set aside.
Add the sweet potato and 1/2 cup water to the skillet and cook over medium heat, stirring, about 5 minutes. Add the pear and cook for 5 more minutes or until the potato is tender. Transfer to a bowl. Toss with the vinegar, mustard, tarragon, the remaining salt, a few grinds of pepper, and 3 tablespoons of water.
Divide the meat or fish among 4 plates. Spoon the vegetables over each entrée. Season with salt and pepper to taste.