Pressure cook the cleaned potatoes with skin on till it's done. I cooked it for 6-7 whistles on full flame.
Once the cooked potatoes are cooled, peel and mash them. It's better if the mashed potatoes have some texture, so make it a bit lumpy. Season the mashed potatoes with salt and pepper. You can also add a knob of butter, if you wish.
Heat oil in a pan and add chopped onion. Once it softens, add sliced mushrooms. Cook till it's done. season it with salt and pepper.
Melt butter in a deep and wide pan. Add minced garlic and cook for 2-3 mins. Add flour and roast till the raw smell goes. Add hot milk gradually and make a thick white sauce. make sure you stir continuously to get a smooth thick white sauce. Add grated parmesan cheese (if using). Season the white sauce with salt and pepper.
Add mashed potatoes and cooked mushroom (keep aside 2 tbsp) to the white sauce and mix well.
Grease an 8-inch baking dish with butter. Transfer the potato white sauce mixture to the greased dish. Level the top with a spatula.
Spread the pizza/pasta sauce on top of the potato mixture. Top the pizza/pasta sauce with breadcrumbs. arrange the kept aside mushrroms on top.
Preheat the oven to 180 C, for 10 mins before baking.
Bake in the preheated oven for 25 - 30 mins.
Let the pie rest at least for half an hour (refer notes). Serve with bread or sausage or salad.
Notes
For added flavor, you can use garlic butter to make the white sauce. You can substitute parmesan cheese with any other cheese of your choice.If you don't have pizza/pasta sauce, you can use regular tomato ketchup. Add some Italian seasoning and chili powder to the ketchup. Ideally, the pie should rest for an hour or so after baking, so that you can slice it perfectly. leftover pie can be refrigerated for 2-3 days and reheat in the microwave before serving.You can also add cooked chicken or bacon pieces along with the mushroom if you like.