Cover the potato with 1" of water in a medium saucepan. Cook until tender, then drain in a colander and coarsely mash it with a fork. Set aside.
Return saucepan to medium heat and melt the butter. Add onion plus 1 tsp salt and cook about 6 minutes, until translucent and just starting to brown.
Add chilli and ginger, cook a minute more, then stir in the spices. After about 30 seconds (when spices smell aromatic) add mashed potato, remaining salt, frozen peas and cilantro; stir until well combined.
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Notes
Use russet potatoes for their high starch content to make your aloo masala mashed, as pictured. Choose yellow or yukon gold potatoes instead if you prefer your potato masala left in cubes.Check your seasoning as potatoes need quite a bit of salt to make the tastiest flavor.