Drop the garlic through food chute with food processor on, and process until minced. Add water and the next 8 ingredients (water through pepper), and process until blended. Combine garlic mixture and chicken in a large zip-top heavy-duty plastic bag, and seal. Marinate in refrigerator 8 hours, turning bag occasionally.
Remove chicken from bag, reserving 1 cup marinade; set both aside.
Heat chicken broth in a large nonstick skillet over medium-high heat. Add onion, cook for 3 minutes or until tender. Add 1/4 cup reserved marinade and tomatoes; bring to a boil, and cook for 5 minutes or until reduced to 2 cups. Remove tomato mixture from heat, and set aside.
Drop Swiss chard leaves into a large saucepan of boiling water; cook 30 seconds. Drain and rinse under cold water; drain well.
Place 1 chicken breast half in center of each Swiss chard leaf, and top with 1/4 cup of tomato mixture. Fold in the edges of leaves, and roll up; place the chicken rolls in a 13 x 9-inch baking dish. Pour remaining 3/4 cup of marinade over the chicken rolls. Cover and bake at 350° for 50 minutes; uncover and bake for an additional 10 minutes. Serve with the remaining tomato mixture. Garnish with cilantro sprigs, if desired.