Place one chicken breast half between sheets of plastic wrap. With the flat side of a meat mallet, pound chicken to 1/4- to 1/2-inch thickness. Repeat with the remaining breast halves.
In shallow dish whisk together egg and buttermilk. In another shallow dish combine almonds, panko, rosemary, and 1/4 teaspoon salt. Dip chicken breasts, one at a time, in egg mixture, turning to coat. Allow excess to drip off, then dip chicken pieces in almond mixture, turning to coat.
In a 12-inch nonstick skillet cook chicken, half at a time if necessary, in hot oil over medium heat for 4 to 6 minutes or until no longer pink, turning once halfway through cooking. Remove chicken from skillet; keep warm.
In the same skillet cook shallot in drippings for 3 to 5 minutes or just until tender, stirring frequently. Add spinach and 1/4 teaspoon salt; cook and toss about 1 minute or just until spinach is wilted. Serve chicken with wilted spinach. Sprinkle with pepper and, if desired, mint.