Season chicken on both sides with salt and pepper. Heat 1 1/2 teaspoons oil in a large heavy skillet over medium-high heat. Add the chicken and sear until well browned, about 3 minutes per side. Transfer the chicken to a plate and tent with foil.
Reduce heat to medium. Add the remaining 1 1/2 teaspoons oil to the pan. Add shallots and red pepper. Cook, stirring, for about 2 minutes. Remove from heat. Whisk broth, grapefruit juice, 1 tablespoon mint and flour in a small bowl. Add to the pan. Cook, whisking, until slightly thickened, about 3 minutes.
Return the chicken and any accumulated juices to the pan; reduce heat to low. Simmer until the chicken is cooked through, about 4 minutes. Transfer the chicken to a warmed platter. Season the sauce with salt and pepper and spoon over the chicken. Garnish with the remaining 1 tablespoon mint.