Cut tempeh into 1" strips and place in baking dish.
Pour barbecue sauce over and top with sliced onions. Marinate in refrigerator for 1 hour or more. A healthy prepackaged oil-free barbecue sauce option is Bone Suckin' Sauce, available through amazon link below. If you prefer to make your own, there's a link below for that too.
Heat oven to 350 degrees or light grill.
Cook until browned and heated through. Baste frequently with barbecue sauce. If cooking in oven, bake for 30 minutes.
Serve warm with rice and veggies or on sandwiches.