Firstly boil or steam 2 medium to large potatoes (about 185 to 200 grams). you can cook potatoes in a pan or a pressure cooker. i usually steam them in a electric cooker or pressure cooker. 

For pressure cooking, add 1.5 cups water and pressure cook for 5 to 6 minutes or 4 to 5 whistles on a medium flame. also add 1/4 tsp salt while pressure cooking. the potatoes should be completely softened and well cooked

Drain the water and take the cooked potatoes in a mixing bowl. let them become warm. peel and mash them with a potato masher or fork. keep the mashed potatoes aside.

When the potatoes are cooking, rinse or wipe 200 grams button mushrooms. chop them finely, but not very finely, as we want their chunky texture in the cutlets. keep aside.

Chop 1 small onion and keep aside. mix everything and fry them and serve hot