Coat skillet with cooking spray. Salt and pepper chicken then add to hot skillet. Cook 6 minutes on each side. Place in oven to keep warm.
On another burner, add 2 cups broth and 1/2 T olive oil to sauce pan. Bring to boil. Remove from heat, add couscous, zest, parsley and stir. Cover and remove from heat.
Using the chicken skillet, add onion, horseradish and thyme. Cook and stir for three minutes. Add mustard, remaining 1 cup of broth, and half & half. Simmer until liquid is reduced by half (about 5 minutes). Return chicken to skillet and cook 1 more minute.
To serve, scoop 1/3 cup couscous onto plate, put breast on top and cover with pan sauce.