Mix the blend. Stir the paprika, garlic powder, onion powder, oregano, thyme, pepper, cayenne and salt together in a small bowl.
Whisk the glaze. In a bowl, whisk the tomato paste, tamari, maple syrup, vinegar, garlic and water with all of the blend until no lumps of paste remain.
Load the pot. Put the onion, pepper, celery and cubed tempeh in a 4-quart (3.8 liter) slow cooker. Pour the glaze over and stir until every cube is coated.
Cook. Cover and cook on low for 4 hours, or on high for 2 hours. The sauce should bubble gently at the edges and the tempeh should look darker and glossy.
Thicken. Stir the cornstarch mixture into the pot, set it to high and cook with the lid off for about 10 minutes. The sauce should thicken to coat the back of a spoon.
Taste and adjust. Add a pinch of salt if needed, or a squeeze of lime for brightness. Let the pot sit off the heat for 5 minutes.
Serve. Spoon over rice and top with sliced green onions and lime wedges.