Soak both the rice in six cups of water for at least four hours. Soak split black grams with fenugreek seeds in three cups of water also for at least four hours. Drain and grind the rice and split grams separately to a smooth batter with dropping consistency.
Add salt and mix both the batters thoroughly with hand in a whipping motion. Keep the batter in a large vessel, close tightly and rest the batter overnight or for about four to six hours at room temperature to ferment.
Take required quantity of batter in a bowl, add salt and sufficient water to get the desired consistency and mix well. The batter should be thicker than that for dosa. Heat a thick tawa or a non-stick pan. Put two drops of oil and wipe the tawa clean with a wet muslin cloth.
Add a tablespoon of butter to the tawa and pour half a ladleful of batter and spread it into a three-inch diameter round. Make as many as would fit on the tawa. Cook on low heat for three to five minutes.
Flip side and cook further, if desired. Apply chutney on one side of each uttapam. Take one uttapam, place a few slices of onion, tomato and cucumber over the chutney. Place the other uttapam, chutney side down over it.
Press gently, cut into wedges and serve immediately. Similarly prepare other uttapam sandwiches.