Nicole Perdue's Slow Cooker Barbecued Turkey Breast

3 pounds turkey breast tenderloins, cut crosswise into 1/4-inch-thick slices (you can also use chicken breast)
2 cups chopped onions (4 medium)
2 cups chopped green bell peppers (2 medium)
⅓ cup packed brown sugar
2 tablespoons all-purpose flour
1 teaspoon salt
1½ teaspoons dry ground mustard
4 teaspoons chili powder
¼ cup vinegar
1 tablespoon Worcestershire sauce
1 6-ounce can tomato paste