There are a few things you can keep in mind while making rajma recipe to make the thick gravy. 1. Make sure to food processor or chop the onions very finely (don’t keep big chunks). Or you can grate the onions.  2. Simmer on LOW for good 10-15 minutes.  3. While simmering the gravy mash a few beans using the back of the spatula which helps in thickening the gravy.