Score one side of meat. Sprinkle with salt and pepper.

Combine green pepper, onion, 1 T tomato paste, horseradish, sugar, 1/2 cup water, and stuffing. Spoon stuffing mixture down middle of unscored side of flank steak. Fold ends over and overlap long sides; skewer together. Place on metal rack or trivet in crockpot. Cover and cook on low for 8 to 10 hours. Remove meat. Turn control to high. Dissolve cornstarch in 1/4 c cold water; stir in 1 T tomato paste. Add to juices in pot. Cook on high for 15 to 20 minutes or until slightly thickened. Slice steak across the grain. Pass sauce with the steak.