Combine the onion, garlic, sweet potatoes, chickpeas, diced tomatoes, olives, vegetable broth, brown sugar, cumin, curry powder, paprika, cinnamon, turmeric, cayenne, salt, and pepper in a large freezer bag. Squish to combine.
Remove excess air, seal, and freeze.
Thaw.
Pour the contents of the bag into a large stock pot or Dutch oven. Bring the liquid to a boil, then reduce heat and simmer until the onions are tender, between two and four hours.
Thaw. Cook on high for four hours.