Rinse split peas and lentils thoroughly; drain.
Bring 8 cups water to boil in large pot. Add lentils, split peas, onion, 2 diced garlic cloves, turmeric, salt, black pepper, and hot pepper (if using).
Reduce to a gentle simmer with lid slightly ajar. Cook about 80 minutes (or 15 minutes in pressure cooker), stirring occasionally and skimming any foam.
Once peas are soft but still holding shape, whisk gently to break down some into a thick stew.
In a small frying pan, heat oil. Add remaining 2 garlic slices and fry until edges darken.
Carefully pour hot garlic oil into dhal over the garlic. Stir and taste. Adjust salt as needed.
Serve hot with rice, roti, or as a bowl of soup.