To make the ancho paste, roast the ancho chile pepper in a cast-iron or other skillet, pressing down with a spatula. Roast each side for a couple of minutes, then remove to a blender along with the tomato paste and the roasted cumin seeds. Blend with enough water to make a thick, smooth paste and set aside.
In a saucepan, heat the oil and add the onion, carrot, celery and garlic. Season with salt and black pepper and cook 5 minutes or until the onions are soft. For an oil-free version, water-saute the veggies, or saute in vegetable stock.
Once the onions are soft, dd the lentils and 3 cups water. Stir well, bring to a boil, cover and cook 15 minutes or until the lentils are perfectly tender but not mushy. Add more water if the mixture dries up.
Stir in the ancho paste and add salt to taste. Let the stew cook another five minutes for flavors to merge.
Blend the cashews with 1/4 cup water and drizzle over the lentils. If you'd like a nut-free version, use coconut milk instead.
Garnish with parsley, if you wish, and serve hot or at room temperature.