Heat the oil and butter in a 7-quart Dutch oven or large covered saucepan over medium-high heat. When the butter foams, stir in the sliced onions and salt, cover, lower the heat to medium-low and sweat for 10 minutes.
Remove the cover, raise the heat to medium-high and continue to cook the onions. Keep a measuring cup of cold water nearby to use in small doses to dissolve the fond that accumulates on the bottom of the pan. This colors and flavors the onions as they soften. The onions are ready when very soft and golden brown, about 20 minutes.
Stir in the carrots, celery, peppers, curry powder and garlic. Once the aroma of the curry is fragrant, add the stock and the lentils. Bring to a boil, lower the heat to a gentle boil and cook for 15 minutes.
Stir in the kale; if it looks like it won’t fit, don’t worry, it wilts quickly. Continue cooking another ten minutes.