Heat 2 teaspoons oil in a large nonstick skillet over medium-high heat. Add onion to pan; sauté 3 minutes or until tender. Add potato, carrot, 1/4 teaspoon salt, and 1/8 teaspoon pepper; sauté 12 minutes or until vegetables are tender and begin to brown. Place potato mixture in a large bowl; keep warm.
Heat remaining 1 teaspoon oil and butter in pan over medium-high heat. Add mushrooms, remaining 1/4 teaspoon salt, and remaining 1/8 teaspoon pepper to pan. Sauté 10 minutes or until mushrooms are tender. Add mushroom mixture and parsley to potato mixture; stir to combine.