I started adding some celery with the onion in the earliest stage. It thickens things a bit. A gumbo recipe will have you cook celery in its own for a long time to get to an almost paste-like consistency, because it provides a lot of thickness and scaffolding to the rest of the gumbo, but for vegetarian chili I want a mixture of textures, so I only cook the celery about as long as I cook the onions.