Clean the crab well. You can refer this link to see to how to clean a crab.

Make sure you remove all the dirt from the crab. Break its legs, do not pop out its flesh while breaking.

For the masala preparation:

Heat oil in a pan. Splutter mustard seeds and add sliced onions, green chilies, curry leaves, and a little salt. Saute until onions turns golden brown.

Make a paste of ginger and garlic. Add this paste to the pan and mix well.

Add chopped tomatoes and place the lid on to cook on a low flame until it turns mushy.

Add spice powders; chili, coriander, turmeric, pepper, and garam masala, and mix well. Make sure you turn the heat to low while adding the spice powders, so that they do not burn.

Add the crab and mix well with the masala. Add three quarters of a cup of water and cook on a medium flame for 5 -8 minutes. Toss the pan carefully while cooking.

Cook until gravy thickens. Garnish with curry leaves and allow it to rest for a few minutes.