Combine 1/4 teaspoon salt, 1/4 teaspoon pepper, and allspice, stirring well; sprinkle evenly over both sides of chicken.
Melt butter in a large nonstick skillet over medium-high heat. Add chicken to pan; cook 3 minutes on each side or until browned. Remove chicken from pan. Add onion to pan; sauté 4 minutes or until tender. Add garlic; sauté 1 minute, stirring frequently. Stir in brandy; cook 30 seconds or until liquid evaporates. Stir in remaining 1/4 teaspoon salt, remaining 1/4 teaspoon pepper, broth, and sage; bring to a boil. Return chicken to pan. Cover, reduce heat, and simmer for 12 minutes. Arrange plums in pan; cook for 8 minutes or until chicken is done. Drizzle with juice.