In 10-inch nonstick skillet, heat oil over medium-high heat. Add chicken thighs; sprinkle with salt and pepper. Cook about 5 minutes or until browned. Reduce heat to medium; add mushrooms. Cook 8 to 12 minutes, turning occasionally, until mushrooms are tender. Remove chicken and mushrooms from skillet; drain if necessary. Cover to keep warm.
To same skillet, add gravy mix and water. Heat to boiling over medium heat, stirring frequently with wire whisk. Stir in cooked chicken and mushrooms. Reduce heat to low; cover and cook 10 to 15 minutes or until sauce is thickened and juice of chicken is clear when thickest part is cut to bone (180°F).
Meanwhile, cook potatoes as directed on box. Serve chicken and mushrooms with mashed potatoes.