Heat the olive oil in a large skillet over medium-high heat.
Add the diced onion and sliced celery. Cook, stirring often, until the vegetables begin to brown, about 5 minutes.
Add the minced garlic and cook for about 30 seconds, just until fragrant.
Remove the skillet from the heat and stir in the chili powder, cumin, and dried oregano.
In a large bowl, combine the cooked white rice, pinto beans, corn, diced tomato, minced chipotle pepper, and the onion-celery mixture.
Season the filling with salt and black pepper to taste, then stir until everything is evenly combined.
Cut the tops off the bell peppers and remove the seeds and pith.
Spoon the rice mixture into each pepper, packing it lightly so the peppers stay full but not tightly compressed.
If using pepper jack cheese inside the peppers, fill each pepper halfway, add a little shredded cheese, then continue filling to the top. Do not add cheese on top yet.
Place the stuffed bell peppers upright in the slow cooker.
Pour water around the peppers until it reaches about 1/2 inch up the sides, being careful not to pour water into the peppers.
Cover and cook on low for 3 1/2 to 4 hours, until the peppers are easily pierced with a fork but still hold their shape.
If you prefer peppers with a little more bite, cook closer to 3 1/2 hours. For softer peppers, cook closer to 4 hours.
During the last 15 minutes of cooking, add shredded pepper jack cheese on top if desired. Cover again and cook until melted.
Remove the peppers from the slow cooker with tongs.
Spoon 1 to 2 tablespoons of enchilada sauce over the top of each pepper and serve immediately, with extra enchilada sauce and optional cheese on the side.