Put the lentils and grains in a sieve and rinse under running water. Dump into the slow cooker and add the potatoes and
In a skillet or sauté pan, heat the ghee over medium-high heat. When it shimmers, add the onion, garlic, ginger, mustard seeds, and cumin seeds and cook, stirring, until the onion is translucent, about 5 minutes. Scrape into the cooker and add the chiles. Scrape the peel off the turmeric root (use a small spoon for this), then finely grate it into the cooker. Cover and cook on low for 8 hours.
Season the dal with salt and stir in the lemon juice. Serve.