PEEL oranges; cut into segments over medium bowl, reserving juice in bowl. Mix 2 Tbsp. orange juice and mayo in pie plate. Mix cracker crumbs and nuts in second pie plate. Discard remaining orange juice; place orange segments in bowl.
DIP chicken in mayo mixture, then crumb mixture, turning to evenly coat both sides of each breast. Heat 1 Tbsp. oil in large skillet. Add half the chicken; cook 8 to 10 min. or until golden brown on both sides and done (165ºF), turning after 5 min. Remove chicken from skillet; carefully wipe skillet with paper towel. Repeat with remaining oil and chicken.
ADD tomatoes and onions to oranges; mix lightly. Serve spooned over chicken.