Soak the Parboiled Idli Rice and the Whole Urad Dal together with the Fenugreek Seeds in plenty of Water for 4–6 hours (or overnight).
Drain and grind the dal and fenugreek mixture to a smooth, fluffy paste using a little of the reserved Water; then grind the rice to a slightly coarse batter and combine both to form a thick, pourable batter. Adjust consistency with more Water so it holds shape but spreads slightly.
Cover and ferment the batter in a warm place until it has risen and is slightly bubbly (6–10 hours, depending on temperature).
While the batter ferments, finely chop the Onion, Tomato, Green Chilies and Coriander Leaves and mix them together for the topping.
Once fermented, stir the batter gently and season with Fine Salt to taste; do not overmix so you retain the airiness.
Heat a nonstick skillet or cast-iron griddle over medium heat and brush lightly with Oil.
Spoon a ladleful of batter onto the hot pan and spread it gently into a thick circle (thicker than a dosa) — you want a pancake-like disc about 1/4–1/2 inch thick.
Immediately sprinkle the chopped onion, tomato, green chilies and coriander evenly over the batter and press them in lightly so they adhere.
Drizzle a little more Oil around the edges and on top; cook on medium heat, covered, until the bottom is golden and the top looks set and slightly bubbly (3–5 minutes).
Flip the uttapam carefully and cook the other side briefly (1–2 minutes) until nicely golden and cooked through.
Remove to a plate and serve hot with coconut chutney, tomato chutney or sambar. Repeat with the remaining batter, adjusting heat as needed so each uttapam cooks evenly.
