In a blender mix the tomato paste, tamarind, shallots, and sherry vinegar. Slowly add the oil. Peel the red onion into thick slices and rub with olive oil. Broil for 5 minutes per side or until tender. Toss with balsamic vinegar to separate onion pieces. Toss the spinach with the baby butter beans and the dressing. Place the grilled onions on top.