Preheat your oven to 180°C (350°F). Place the whole peppers on a baking sheet and roast for about 15 minutes, turning occasionally until their skins are slightly blistered and softened. Remove from the oven and set aside to cool slightly.
While the peppers cool, prepare the filling by mixing cooked grains, chopped vegetables, herbs, minced garlic, and olive oil in a bowl. Season with salt, pepper, and a squeeze of lemon juice, then stir until well combined.
Carefully slice off the tops of the roasted peppers and remove their seeds and membranes with a spoon or your fingers, being gentle to keep their shape intact.
Stuff each pepper generously with the prepared grain mixture, pressing down slightly to pack the filling in evenly.
Arrange the stuffed peppers upright in a baking dish, then cover loosely with aluminum foil to trap moisture and help them cook evenly.
Bake the peppers for 25 to 30 minutes, until they are tender and slightly blistered around the edges. The filling should be heated through and fragrant.
Uncover the dish and bake for an additional 5 minutes to allow the tops to brown lightly and develop a slight crisp.
Remove from the oven and let the peppers rest for 5 minutes, allowing the flavors to meld and making them easier to handle.
Serve the stuffed peppers warm, drizzled with a little extra olive oil or lemon juice if desired, and enjoy their vibrant colors and cozy flavors.