Drizzle a tablespoon of extra virgin olive oil over the chopped vegetables (onions, garlic, bell peppers). Transfer to an oven-proof dish or tray lined with parchment paper.
Preheat oven to 180 degrees (fan-assisted) or 200 degrees. When hot, roast vegetables for 10 minutes.
Transfer to a blender, add the tomato sauce and water, and blend to a puree.
Empty contents of blender into a saucepan. Add the sugar, rice vinegar, fish sauce and salt and bring to a boil over medium to high heat. Boil for one minute then reduce the heat to medium and simmer until the sauce thickens. This should take about 3 - 4 minutes.
Combine the cornstarch and water, whisk to mix thoroughly, then add to the saucepan to thicken the sauce further.
Let cool completely before storing in a glass jar and refrigerate
