Preheat your oven to 375°F (190°C). Wash and dry the bell peppers, then carefully cut off the tops and carve jack-o'-lantern faces into each using a paring knife. Set aside the tops for later use.
Scoop out the seeds and membranes from inside each carved pepper to create a hollow cavity suitable for stuffing. Place the peppers upright on a baking sheet lined with parchment paper.
In a mixing bowl, combine the cooked rice, diced vegetables, shredded cheese, and minced garlic. Season with salt, pepper, and a splash of balsamic vinegar if using. Mix well to create a flavorful filling.
Use a spoon to fill each carved pepper with the rice mixture, packing it gently but firmly. Place the carved tops back onto each pepper to resemble the “hat” of the jack-o'-lantern.
Bake the stuffed peppers in the preheated oven for about 35-40 minutes, until the peppers are tender and the filling is bubbling around the edges. The carved faces will become slightly roasted and charmingly carved.
Remove from oven and let cool slightly. Carefully transfer the peppers to a serving platter, then enjoy their colorful appearance and savory aroma. Serve warm for a fun and festive main dish.