Wearing gloves, carefully cut the top off of the poblano peppers, creating a lid (as if you were prepping a pumpkin for carving). Remove the seeds and veins from inside the peppers.
In a large bowl, mix together black beans, cooked rice, diced tomatoes, minced onion, cilantro, salt, and cumin. Add minced jalapeno, if using. Mix all ingredients well. Add cheese, and mix until combined.
Stuff each pepper with rice mixture, leaving enough room at the top for the ‘lid’ you created earlier to return to its place.
Lay the stuffed, capped peppers on a foil-lined baking sheet. Bake for 30 minutes, checking and turning every 10 minutes or so, to be sure they are not burning.
Serve warm, with salsa and sour cream.
Notes
Pobalanos, while they can be spicy when raw, lose much of their heat during cooking. For a spicier stuffed pepper, you can serve the peppers after about 20 minutes, while the peppers' flesh is still slightly crisp. For a milder pepper, cook until the peppers are soft.