There are a few tips that you can follow to make curry thick. 1. Finely chop the onions and saute till light brown color. Later when the onions cook in gravy, it almost turns into a paste. 2. Use tomato puree. It helps to thicken the gravy and texture is best. 3. Simmer the gravy for good 15-20minutes. 4. Mash a few rajma while simmering the gravy. The starch from the beans will help in thickening the gravy.