Add oil to large soup pot. Heat to medium-high heat. Add onions, stirring to coat, then cover pot and reduce heat to medium-low to sweat the onions for about 5 minutes. Meanwhile, chop the eggplant and mince the garlic, then add to onions. Place lid on pot. Increase heat to medium. Cook for a few minutes, until the eggplant softens. Chop the other vegetables and add. When the carrots, celery and mushrooms just soften, about 3 minutes, add the tablespoon of flour. Stir well and make sure the vegetables are coated. Pour in the ale.
Mix well then add the sage and celery leaves. Cover and reduce heat to low. Simmer for an hour and a half, coming back and give the whole thing a stir now and again when you've a mind to.
After 90 minutes, remove the pot lid, bring heat back to high. The eggplant will be meltingly soft. Bring to a boil and boil for about 3 minutes to reduce sauce and concentrate the flavor. Stir in nutmeg, grated orange rind and salt and pepper to taste.
Serve over rice (and since it's me talking here, I'd recommend whole grain brown rice over traditional white). Nice with a green salad. Stew flavor improves over time. It'll taste better tomorrow, if you have any left.