Step 1 — Load the Slow Cooker
Add the dried brown lentils, diced onion, sliced carrots, diced celery, minced garlic, ground cumin, and ground coriander directly into your slow cooker. Pour in all 8 cups of vegetable broth. Stir everything together until the spices distribute evenly throughout the liquid and vegetables.
Cover the slow cooker and turn it on. Cook on HIGH for 2 1⁄2 to 3 1⁄2 hours, or LOW for 5 to 6 hours, until the lentils are completely soft and the vegetables are fully tender.
The cook time range accounts for the variation between different slow cooker models. Start checking at the 2.5-hour mark on HIGH or the 5-hour mark on LOW. The lentils should crush easily between a spoon and the side of the pot when they're done.
Step 2 — Add the Diced Tomatoes
Once the lentils reach full tenderness, stir in the can of diced tomatoes — liquid and all. Replace the lid and let the dal cook for an additional 20 minutes to heat the tomatoes through and allow their flavor to meld into the stew.
Why add the tomatoes at the end rather than the beginning? Tomatoes contain acid, and acidic ingredients slow down the softening of legumes during cooking. Adding them at the start could leave you with firm lentils after the full cook time. Adding them at the end avoids this entirely while still giving them enough time to contribute their flavor. :/
Step 3 — Season and Serve
Season the dal generously with salt and pepper to taste. This is the moment where you finalize the flavor — lentils need proper salting to taste their best, so don't rush through this step. Taste as you go. Start with ½ teaspoon of salt, stir, taste, and adjust from there.
Ladle into bowls and serve immediately. The dal thickens considerably as it sits, which is completely normal and desirable.
