In a large flameproof dish, combine the peanut butter, groundnut oil, half the lemon juice, the onions, chillies, garlic, thyme, bay leaf and the pimentos. Season with salt and pepper. Place tofu in this marinade and leave for 30 minutes.
Pre-heat the grill to high for 5 minutes. Place the dish of marinated tofu under the grill until it has browned on top. Turn and brown again, then twice more until the tofu is golden brown all over. Serve with rice.