Set your oven to 375 F. Cut the onion in half, peel the cloves of garlic, and chop your carrots into thirds or quarters.
Drizzle with olive oil and put in to roast. Remove garlic after 15 minutes and let the carrots and onions go for about 40 minutes. (Shorter is fine if you are in a pinch, but a longer roast will bring out more sweetness.)
Add both cans of tomato and about 1 tbsp of each spice to the blender, along with 1 tbsp of olive oil and 1 tbsp of fish sauce. When the veggies are done roasting, add them too. Blend until it is the consistency you prefer.