Bring 2 cans chicken broth to a boil. Add chicken to the broth, cover and reduce heat to simmer. Poach chicken 12 minutes. Remove chicken to a plate and pour broth into a large measuring cup or batter bowl. Return the pan to heat and add butter. When butter melts, add flour and cook, whisking with butter, 2 minutes. Slowly pour cooking liquid back into the pan, until appropriately thickened, combining with a whisk. Stir in Dijon mustard and tarragon and season sauce with salt and pepper. Return chicken to the pan and coat with sauce. Simmer 2 or 3 minutes to heat chicken back through and to combine flavors. Serve with rice pilaf, and roasted squash vegetable medley.