Cook grains for filling: You can use rice, quinoa or your favorite grain blend. Cook according to package directions.
Prepare poblano peppers: Set the oven to broil. Arrange peppers on a baking sheet lined with foil. Broil peppers for 5-7 minutes, turning over if desired until it begins to char in spots -- the skin will turn black and blister.
Remove from oven and place in a deep dish or bowl; cover with plastic wrap or foil and set aside for about 10 minutes. This allows the peppers to steam and will make it easy to remove the papery outside skin of the peppers.
Once the peppers are cool enough to handle, carefully peel off the charred outer skin and discard. Make a long slit in each pepper, making sure not to cut through to the other side. Carefully reach in and remove all the seeds and any tough membranes and discard.
Make Filling: In a small bowl, whisk together olive oil, cumin and lime juice. Add to cooked grains and stir to combine. Fold in black beans and shredded cheese. Set aside.
Prepare baking dish: In a medium baking dish, pour the jarred salsa. Set the oven temperature to 375 degrees.
Fill peppers: Gently scoop the filling into each pepper until full {about ⅓ cup should do it}. Nestle each stuffed poblano into the sauce at the bottom of the baking dish.
Cover with foil and bake for about 15-20 minutes to allow the cheese in the filling to melt and the sauce to heat through. Remove the foil in the last few minutes of baking and add an extra sprinkle of cheese on top. Top with diced avocado and cilantro before serving if desired.