Split tempeh in half horizontally with a sharp knife; cut each piece in half crosswise. Place tempeh in a heavy-duty zip-top plastic bag. Combine water and next 5 ingredients (water through garlic) in a small saucepan; bring to a boil. Pour over tempeh; seal bag. Marinate in refrigerator 3 hours to overnight.
Place buns, cut sides down, on grill rack; grill 1 minute or until toasted.
Remove tempeh from marinade, reserving marinade. Place tempeh on grill rack coated with cooking spray; grill 2 minutes on each side or until lightly browned, basting frequently with reserved marinade. Spread 1 1/2 teaspoons ketchup and 1 1/2 teaspoons mustard evenly over each bun top. Place 1 lettuce leaf on bottom half of each bun; top with 2 tempeh pieces, 1 tomato slice, 1 onion slice, and top half of bun.