For the Pickled Onions: In a small microwave-safe bowl mix the water, vinegar, sugar, and salt. Microwave for 60-90 seconds to dissolve the sugar. Slice the onions thin. Then mix them into the pickling water. Allow them to soak as you make the lentil curry.
For the Red Lentil Curry: Chop the onions, carrots, garlic and grate the ginger. Set a large 6-8 quart pot over medium heat. Add the oil.
Once hot, add the onions, carrots, garlic, ginger, 2-5 tablespoons curry paste, cumin, and salt. Saute until the onions are soft and tender. *If your curry paste tastes/smells very spicy, add only 2 tablespoons. You can always add more later.
Pour in the dried red lentils, chicken broth, crushed tomatoes, and coconut milk. Stir well. Simmer for 20 minutes, stirring every 3-5 minutes, so the lentils don't stick to the bottom of the pot.
If the liquid seems to absorb before the lentils are fully cooked, add some water to loosen up the mixture. After 20 minutes, the lentil curry should be soft and thick.
Drain the pickled onions. Serve the lentil curry warm, on its own, with rice, or with baby spinach. Top with pickled onions, cashews, and cilantro leaves.
Notes
Curry paste brands vary greatly in salt and spice. Taste the curry paste to determine how much you want to use. I recommend 2-5 tablespoons. If gluten-free is important to you, also check the curry paste label. Most are gluten-free, but not all.