To make the sauce, in a large saucepan or Dutch oven, heat the oil and sauté the onion and garlic until softened and transparent, about 5 minutes.
Add the tomatoes, maple syrup, molasses, vinegar, mustard powder, chile powder, salt, pepper, and paprika to the saucepan.
Bring to a boil and reduce the heat to low.
Cook, uncovered, for 1 hour. Puree the sauce, using an immersion blender, or in batches in a blender or food processor.
Return the sauce to the saucepan and continue simmering for 30 minutes more. Remove from the heat and let cool.
Prepare the wooden skewers by soaking them in water for 15 minutes.
Preheat the grill to medium heat. Skewer the vegetables and tempeh in a colorful pattern.
Brush the kebabs generously with the sauce.
Grill the kebabs for 3 minutes on each side. You may need to brush the kebabs with more sauce after flipping to the other side.
The kebabs are ready when the vegetables are slightly browned and seared. Serve over rice.
Leftover sauce can be stored in an airtight container for up to 2 weeks in the refrigerator or 2 months in the freezer.