1. Place rack in top position of oven, and preheat oven to 450°F. Arrange peppers on rimmed baking sheet, and roast 10 to 12 minutes, or until slightly softened and wrinkled. Cool on baking sheet.
2. Lay 1 pepper on its side, and use paring knife to cut long, oval-shaped opening into side of pepper, leaving stem and bottom intact. (Pepper will resemble a canoe.) Reserve oval-shaped piece for filling. Carefully scoop out and discard seeds from body of pepper. Repeat with remaining peppers. Chop up oval-shaped pieces, and set aside.
3. Place rice in saucepan, cover with salted water, and bring to a boil. Reduce heat to medium, and cook 15 minutes. Drain.
4. Meanwhile, whisk together oil, vinegar, oregano, and mustard, and season with salt and pepper, if desired. Stir mixture into hot rice, then mix in reserved chopped peppers, cherry peppers, and cheese.
5. Stuff mini peppers with rice mixture, and arrange on platter.