Preheat oven to 210℃/410℉.
Prepare the capsicum/peppers. For bell peppers, remove seeds and slice thinly. For small, sweet peppers, simple remove core by slicing from top.
Slice onion into strips.
Put capsicum/peppers in baking dish. Add garlic, sugar, olive oil. Mix so vegetables are well coated. It's a bit of a visual at this point, you can add a little more oil or sugar if you think it needs it.
Roast for 25-30 minutes, turning occasionally, until cooked and browned. Serve with any remaining oil/sugar mix. Can be eaten hot or cold.