Bring 2 cups of chicken broth to a boil. Add Chicken to the broth, cover and reduce heat to simmer. Poach 12 minutes. Remove chicken to a plate and pour broth into a large measuring cup. Return pan to heat and add butter. When butter melts, add flour and cook, whisking with butter 2 minutes. Slowly pour cooking liquid into the pan, until appropriately thickened, combining with a whisk. Stir in 1 tablespoon dijon mustard and tarragon and season sauce with salt and pepper. Return chicken to pan and coat with sauce. Simmer 2 to 3 minutes to heat through and to combine flavors. Serve with rice pilaf.
