Brown ground beef in large pot on medium heat. Drain off grease. Stir in tomato paste and cook for 1 minute.
Add onions, celery, and garlic. Cook 5 minutes, stirring occasionally.
Next, add mushrooms and bell peppers, and cook for another 5 minutes.
Stir in the remaining ingredients, except for the green onions. Simmer on medium-low partially covered for about 15 minutes, stirring occasionally. The chili should start to thicken.
Adjust seasoning to taste and simmer covered for another 10-15 minutes until the vegetables are tender, stirring occasionally.
Turn off heat, stir in green onions and let sit covered for 10-15 minutes (prepare the cornbread crust during this time).