1. Heat oven to 400 degrees F.
2. Heat 1 tablespoon of oil in a nonstick skillet over medium-low heat. Add mushrooms and leeks; cook, stirring, 4 minutes, or until tender. Remove from heat; sprinkle with half the thyme, salt and pepper.
3. With a paring knife, cut a small pocket in the top edge of one side of each chicken breast, being careful not to cut through the breast.
4. Fill with 2 to 3 tablespoons of the mushroom filling. Season chicken with remaining thyme, salt and pepper.
5. Heat remaining 1 tablespoon oil in an ovenproof skillet over medium-high heat. Brown chicken on both sides.
6. Transfer pan to oven and roast 10 minutes, or until chicken is cooked through. Serve with remaining mushroom-leek mixture.