Place the onion, garlic, and ginger in a food processor, and process into a chunky pulp. (Or, finely dice the onion and mince the garlic and ginger by hand.)
In a large Dutch oven or pot, heat 1 tablespoon of water on medium heat. Add the onion , garlic and ginger, and saute for 5 minutes or until soft and fragrant. Add water a little at time if things begin to stick.
Add the lentils, and stir to heat through. Add the stock and water, turn the heat up to high, and bring to a boil. When just beginninng to boil, reduce the heat to medium low, cover, and simmer 12-15 minutes or until lentils are very soft.
While the lentils are simmering, temper the seeds. Heat a dry (no oil or water) saute pan on medium high heat. When hot, add the cumin seeds and brown mustard seeds. Heat for a few minutes until they pop and are fragrant, being careful not to burn them. (I cover the pan with splatter screen to prevent the seeds from popping out. )
When the lentils are cooked, blend about half of the dal in a blender (or use an immersion blender). Add the blended soup back to the pot. (This step is optional, but I recommend it.)
Add the tempered seeds and the fresh lemon juice to the pot, stir, add sea salt to taste, and serve.