Coat the bottom of a medium saucepan with oil and place it over medium heat. When the oil is hot, add the onion and sauté until soft and translucent, about 5 minutes. Add the garlic and sauté until very fragrant, about 1 minute more.

Add the water, tomato past, apple cider vinegar, molasses, soy sauce, maple syrup, chili powder, and cayenne to the pot. Stir until everything is well blended, then stir in the sweet potato. Raise heat to high and bring the liquid to a simmer. Lower the heat and allow to simmer, uncovered, for 15 minutes, stirring occasionally.

Stir the chickpeas into the pot. Allow everything to simmer until the sweet potato is tender and the sauce is thick, 5 to 10 minutes more. You can add a splash of water during cooking if the sauce gets too thick. Taste test and adjust the seasonings to your liking.

While the barbecue simmers, place polenta grits, 2 cups of water, and milk into a medium saucepan and stir a few times. Place over medium heat and bring to a simmer. Lower heat and allow to simmer, uncovered, for about 15 minutes, stirring occasionally and adding a bit of extra water if mixture becomes too thick. Remove from heat and allow to sit for 5 minutes. Season with salt and pepper to taste.

Divide the polenta into bowls, then top with the chickpea and sweet potato barbecue. Drizzle with tahini and sprinkle with scallions. Serve.