Heat a four-quart heavy-bottomed pot over medium heat. Toast coriander, fennel, and mustard seeds until fragrant, about 1 minute. Transfer to a spice grinder and process until finely ground. (You can also crush the seeds with the side of a chef's knife.)
Heat oil in the pot. Add ground seeds, cinnamon, chili, garlic, onion, ginger, and curry leaves (if using). Cook, stirring occasionally, until onions are soft, 6 to 8 minutes.
Add tomatoes and bring to a boil, then simmer, stirring occasionally, until slightly thickened, about 15 minutes.
Add potatoes and 2 cups of water. Simmer, partially covered, until potatoes are tender, about 35 minutes.
Add peas and cook until tender, about 12 minutes.
Season with salt. Stir in yogurt. Remove cinnamon stick before serving.