Preheat your oven to 425°F / 220°C and ensure the oven rack is positioned in the center.
Wash the 1. 5 lbs / 680g (24 oz) of green bell peppers, remove the seeds and white membranes, and chop them into roughly 2-inch / 5 cm pieces.
Scrub the 1 lb / 454g of small red potatoes thoroughly to remove any dirt, then cut them into 1-inch / 2. 5 cm slices or chunks.
Peel the large yellow onion and cut it into large chunks, keeping the size relatively consistent with the peppers for even roasting.
Transfer all the chopped vegetables into a large, shallow ovenproof dish or a rimmed baking sheet.
Pour the 0. 25 cup / 60ml of extra-virgin olive oil over the vegetables and toss well with tongs or your hands to ensure every piece is coated.
Sprinkle the vegetables evenly with 1 tsp / 6g (0.21 oz) of kosher salt and 0. 5 tsp / 1g (0.04 oz) of freshly ground black pepper.
Bake for approximately 30 minutes, giving the vegetables a stir halfway through, until the potatoes are fork-tender and the edges of the onions are golden brown.
Don't Overcrowd the Pan: This is the golden rule of roasting. If the vegetables are piled on top of each other, they will steam instead of roast. Use two baking sheets if necessary to spread them out in a single layer for that delicious, charred edge.
Add Herbs at the End: If you want to add fresh herbs like parsley or basil, sprinkle them on after baking. If you prefer woody herbs like rosemary or thyme, toss them in with the olive oil before baking for an aromatic infusion.