Season chicken with 1 Tbsp. olive oil, salt, and pepper.
In a large skillet over medium heat, add remaining olive oil and chicken, making sure not to overcrowd the pan (work in batches if needed). Cook 5–7 minutes per side, until mostly cooked through but not fully done. Remove chicken and set aside.
In the same pan, melt butter and sauté onion, garlic, thyme, and mushrooms for 7–10 minutes, until fork-tender. Remove from pan.
Prepare gravy mix in the same skillet according to package directions.
Add mushroom mixture and chicken back to the pan and cook on low for 10 minutes. Serve warm over mashed potatoes.
