You can substitute canned tomato puree in this recipe, if you like convenience over the freshness of tomatoes.
Seasoning could be done in the large pan and the tomato and onion puree cooked in the same seasoned oil too. This lessens the usage of multiple pots and pans. But if convenience is not your first priority, it’s best tasty when the seasoning is done in the end, since the flavors of the season remain fresh and flavorful.