In a large non-reactive saucepan heat olive oil. Add onions and carrots and reduce heat to low. Add 1/4 cup of water and garlic. Cover and cook gently until vegetables are tender, about 15 minutes. Stir in tomatoes with puree, using a wooden spoon to break up tomatoes by mashing them against the sides of the saucepan. Add remaining 1/2 cup of water, cover and simmer over low heat for 1 hour, stirring now and then. Press sauce through a sieve or food mill and discard solids. Season to taste with salt and pepper. Use immediately or ladle into freezer containers. This sauce can freeze up to a year.