Knead wheat flour with salt, 1 tsp oil, and water into a firm dough.
Boil, peel, and crush potatoes.
Heat 1 tbsp oil in a pan, add mustard seeds, curry leaves, green chili, and sliced onion.
Add turmeric powder, crushed potatoes, salt, and 0.25 cup water; simmer for 5 minutes.
Roll small dough balls into disks and deep-fry in hot oil until fully puffed.
Serve hot puris with warm potato bhaji.