Some chana masala recipes call for blending the tomatoes and onions into a thick paste before adding the spices and water, creating a "creamier" result that isn't as chunky. I like the results without so I skip this step to save time and washing. However, if you want, you can do so. Simply transfer the mixture to a small processor/blender after cooking down the tomatoes for 10 minutes (before adding the spice powders) and blend into a thick paste. Then continue with the recipe as written.