Measure the lentils into a colander. Sort through them to pick out any small pebbles or debris and then rinse them well under cold running water. Place the lentils in a large stock pot and add the salt and water. Cover and bring to a boil. Turn the heat down to a simmer and cook for about 15 minutes, or until the lentils are tender and just beginning to fall apart.
In a separate pan, heat the butter until it melts. Add the coriander, cumin and mustard and cook for about a minute, stirring often. Add the garlic and ginger and cook for another minute or two until it is very fragrant. Add this spice mixture to the lentils and mix well.
Pull the kale leaves off the stems and discard the stems. Wash the leaves well and then chop into small pieces. Add the kale all at once to the hot lentils. Turn the heat back to medium-high and cook for about 5 to 10 minutes, or until the kale is as tender as you would like it to be.
To make the cucumber yogurt sauce, peel the cucumbers, slice them in half, and scrape out the seeds with a small spoon. Chop the cucumbers into small pieces and put in a medium bowl.
Add the yogurt, garlic, about half the lemon juice, and a few sprinkles of salt. Mix well and taste. Continue to add lemon juice and salt until it tastes good to you.
Serve lentils and kale over brown rice, if desired, and garnish with cucumber-yogurt sauce.