1) To prepare grilled vegetables: Preheat a griddle. Coat vegetables lightly with oil, place on griddle just away from the hottest spot. Turn occasionally with tongs to sear all surfaces. Remove to a dish to let cool. Reserve any juices that accumulate.
2) In a large saucepan, saute onion and garlic with butter over medium heat, stirring. In a food processor, small chop the griddled vegetables. In saucepan with onion/garlic mixture, add vegetable stock, cream, griddled vegetables, and coriander. Bring to simmer, stir to heat evenly. Combine cornflour and water in a small separate bowl. Slowly add to saucepan while stirring to combine. The sauce will thicken slightly in 1 to 2 mins to desired consistency. Add more cornflour or water if necessary.
3) Add salt, dash with pepper, to taste. Serve with suggested garnishes, if desired.