Combine brown sugar, vinegar, reserved pineapple juice, soy sauce and garlic in small bowl; stir until sugar is dissolved. Set aside.
Melt 1 tablespoon butter in 12-inch skillet until sizzling; add half of steak pieces. Cook over medium-high heat, stirring occasionally, until browned (4 to 5 minutes). Place steak pieces in slow cooker. Repeat with remaining butter and steak pieces.
Add carrots and onion to slow cooker. Pour pineapple juice mixture over vegetables. Cover; cook on Low heat setting for 7 to 9 hours, or on High heat setting for 3 to 4 hours or until meat is tender.
Increase heat setting to High. Dissolve cornstarch in cold water in small bowl. Stir cornstarch mixture, pineapple chunks and green pepper into beef mixture. Cover; continue cooking 30 minutes or until green pepper is crisply tender and juices are thickened.
To Freeze: Freeze in freezer safe container.
To Reheat: Heat in covered pot or in microwave until hot.