Combine first 9 ingredients in a small bowl, stirring with a whisk.
Cut tempeh in half lengthwise; cut slices in half. Brush tempeh slices, bell peppers, and onion with the ketchup mixture. Place on grill rack coated with cooking spray; grill 4 minutes on each side or until the tempeh is thoroughly heated. Remove the tempeh, bell peppers, and onion from grill. Cut bell peppers into 1/2-inch-wide strips; separate onions into rings.
Place 1 tempeh slice on bottom half of each bun. Top each tempeh slice with one-fourth of bell peppers, one-fourth of onion, and top half of bun.