Place red lentils and yellow split peas into a large bowl and cover with water. Let soak for a few minutes and swish to wash the lentils and split peas. Drain well and rinse. Place in a slow cooker and add the 6 cups water. Add the diced tomatoes, onion, garlic, ginger, turmeric, cardamom pods, bay leaf, salt and black pepper.
Heat a small skillet over medium heat. Add all of the seeds to the pan. Shake the pan while on the heat, or use a wooden spoon to stir, while toasting the seeds. When they smell fragrant, remove from heat and transfer to the slow cooker. (It will probably bubble up a bit as the hot seed hit the liquid.) Stir well to combine all ingredients. Place lid on slow cooker and let cook on high for 4-5 hours or low for 8-10 hours.
Remove lid after cooking time and stir. Taste and if lentils are not soft, cook for another 30-60 minutes, if needed. Add more salt and pepper, if needed.
To serve - Ladle the daal over brown rice and top with a squeeze of fresh lemon, cilantro, and more seeds, if desired.
Notes
Be careful not to toast the seeds for too long or the dal may become bitter. For a lighter flavor, use half as much of the fenugreek.
If onion seeds cannot be found, it's fine to leave them out. The same for the fenugreek. The mustard and cumin seeds are fairly easy to find.
Recipe by Cafe Johnsonia at https://cafejohnsonia.com/2014/02/slow-cooker-red-lentil-dal-recipe.html