Slice the tempeh into 6 or 8 rectangles, depending how wide you want the “ribs” to be.
Steam the tempeh for 15 minutes to soften.
Whisk together lemon juice, oil, tamari, chili powder, smoked paprika, garlic, oregano and sririacha in a shallow dish or plate.
Arrange steamed tempeh in dish, turn to coat all over with marinade, cover and refrigerate, turning halfway through, for at least 2 hours or overnight.
Prepare a grill for medium heat cooking.
Grill tempeh, flipping once, until browned and hot throughout about 4 minutes on each side.
Transfer to a plate and serve with barbecue sauce on the side if desired.