Mix the parboiled brown rice and the two dals, cover with at least 2 inches of water, and let them soak for 5-6 hours.
Drain the soaked dals and rice and place in a blender along with the remaining ingredients. Add water and blend into a smooth batter that should be about the consistency of a pancake batter.
Heat a cast-iron or non-stick griddle on a medium flame until a drop of water flicked on the surface skitters and evaporates.
Pour about 1/2 cup of the uthappam batter in the centre and if needed, coax a little with the ladle to form a circle about 5-6 inches in diameter. You can make your uthappam thicker if you like. Either use more batter or shape it into a smaller circle.
Sprinkle on some of the chopped onions and any other vegetables or herbs you're using.
Drizzle a few drops of oil along the edges of the uthappam. This makes for a crisper edge and also makes it easier to flip the uthappam.
Place a lid on top of the pan and let the uthappam cook about two minutes. It is important to cover the pan because if you don't you might end up with semi-cooked batter on the inside and that wouldn't taste any good. You want the uthappam to steam thoroughly on the inside.
Flip the uthappam once it is golden brown on the bottom and cook for about a minute more.
Serve hot with some sundried tomato chutney (recipe follows) or with a traditional coriander-coconut chutney.