1. Heat the oil in a large skillet over medium-high heat. 
2. Add the onion and celery and cook, stirring often, until starting to brown, about 5 minutes. 
3. Add the garlic and cook for 30 seconds or so, then remove from the heat. 
4. Stir in the spices. 
5. Place the rice, beans, corn, tomato, chipotle chili peppers, and onion mixture in a large bowl and stir together. 
6. Add salt and pepper to taste. 
7. Cut the tops off the bell peppers and remove the seeds and pith. 
8. Spoon the rice mixture into each of the peppers, packing lightly.  Note: If you want to add cheese, fill a pepper halfway, add some shredded cheese, then fill to the top. 
9. Do not add cheese on top yet. 
10. Place the bell peppers in a slow cooker. 
11. Fill the area around the peppers with about an inch of water, about 2 cups (be careful not to get the water in the peppers) and cook on low for 3.5-4 hours, until they are easily pierced with a fork but still hold their shape.  Note: If you prefer your peppers with a bit of crunch, go for a shorter time. 
12. In the last 15 minutes of cooking, add cheese on top if you like and cook until melted. 
13. Remove the peppers from the cooker with tongs. 
14. Spoon 1-2 Tbsp. enchilada sauce onto the top of each pepper and serve immediately, with more enchilada sauce and optional cheese on the side. 

Enjoy!