Heat oil in a large nonstick skillet over medium-high. Add mushrooms; cook 6 minutes or until browned, stirring occasionally. Sprinkle flour over pan; cook 1 minute, stirring constantly. Add stock; cook 4 minutes or until slightly thickened. Stir in peas, carrots, salt, pepper, and chicken; cook 2 minutes or until thoroughly heated.
Spoon chicken mixture evenly over toasted bread slices. Sprinkle with parsley.