Butterfly OR slice chicken to make about 12 thin medallions. Dust lightly with flour.
Heat 2 tablespoons oil in a large skillet over medium-high heat. Add chicken and cook for 4 to 5 minutes, turn and cook an additional 3 to 4 minutes. Note: chicken may need to be cooked in batches. Remove chicken and keep warm. Add brandy and cook until brandy is nearly evaporated.
Add remaining 1 tablespoon oil and onions to skillet. Cook 2 minutes. Add cream, Dijon mustard, tarragon and black pepper. Continue to cook until slightly thickened, 3 to 4 minutes.
To serve, place chicken on rice or pasta. Ladle sauce over chicken.