rim about 1/8 inch from the bottom of each pepper so it can stand upright. Cut the top 1/4 inch off each pepper. Remove and discard the stems, then finely chop the remaining flesh from the tops and set aside. Scoop out and discard the seeds and as much of the membranes as you can from inside the peppers.

Mix the reserved chopped pepper with the ground beef, tomatoes and their juices, 1 cup of the cheese, cooked rice, garlic, 3 of the scallions, chili powder, cumin, 1 1/2 teaspoons salt and several grinds of black pepper in a large bowl with your hands until combined. Dividing evenly, spoon the filling into the hollowed-out peppers, packing it in (the filling will mound a bit above the tops).

Pour the chicken broth into the bottom of a 6-quart slow cooker, then add the stuffed peppers. Season the top of each pepper with another pinch of salt. Cover and cook on high until the peppers are tender and the ground beef is cooked through, for 3 to 3 1/2 hours. Top with the remaining 1 cup cheese, cover and cook on high for 10 minutes more to melt the cheese. Sprinkle the peppers with the remaining scallions and add a dollop of sour cream to each.