Place chicken in slow cooker. Drain pineapple, reserving juice. Pour Braggs, pineapple chunks, and 1⁄4 cup pineapple juice (from the can) over the chicken. Sprinkle flakes on top. Cover and cook on high 4 hours or low 6-7 hours. Serve pineapple and chicken over the spinach leaves. Top with red onion and slivered almonds if desired.