Put the carrots, celery and onion in a roasting tray ... Remove it from the roasting tray and check it is cooked through (see tips), then set aside on a platter to rest for 10-15 minutes.
Put the onion, carrots, garlic, bay leaf, butter and half the olive oil into a large pan. Cook over a low–medium heat for about 10 minutes until the vegetables are tender but not coloured.