– 1 lemon, zested and juiced – 3 cloves garlic, minced – 1/2 teaspoon red pepper flakes, or to taste – 1/2 cup chicken broth – 1 1/2 pounds skinless, boneless chicken breasts (about 2) – salt and freshly ground black pepper to taste – 2 tablespoons olive oil – 1/2 cup diced shallot – 1 pound asparagus, cut into 3-inch pieces, discarding any tough end – 1 tablespoon minced fresh basil – 2 tablespoons unsalted butter – 1/3 cup heavy cream – fresh basil sprigs for garnish (optional) – lemon slices for garnish (optional)