Soak urad dal and fenugreek seeds in water for 2-3 hrs.
Soak idli rawa in water separately for 2 hrs.
Drain the water from urad dal and fenugreek mixture and squeeze out extra water from idli rava and keep them separately.
Gring urad dal and fenugreek seeds mixture to a very fine and smooth mixture in wet grinder or mixie.Add little water in between for easy grinding.It takes approximately 15-20 mins.
Remove the urad dal batter in a big vessel and then add squeezed idli rava and salt to the urad dal batter and mix well with your hands.
Keep it aside for little fermentation for 6-7 hrs or over night.For making dibba rotti the fermentation need not to be done completely
Take a thick kadai,I use my Indolium kadai for this as it makes the crust of dibba rotti really crisp.Non-stick ones do not give good results,if you do not have choice you can use them but heavy cast Iron or Indolium or thick brass vessels
Heat the kadai and pour the oil(4-5 tbsp) from the sides of the kadai so that the oil coats all the walls of the kadai.
If you are using non-stick,you need not to worry about it.You can proceed to prepare them with 2 tsp oil.
Now add 2-3 ladles of batter in the kadai,take care not to exceed 1 1/2 " thickness in the center.Cover it with lid and let it get roast on low flame for at least 8-10 mins.If using non-stick check in between,it might take less time.
once you see red color in the edges increase the flame for 1-2 mins and gently slide it and turn to the other side.