Peel the onion and chop it coarsely.
Peel the sweet potato and cut into 1/4 to 1/2-inch dice. You should have about 1 1/2 cups.
Mince the garlic.
Put the frozen kale in a colander and thaw under cold running water; squeeze out as much moisture as possible.
Heat the olive oil in a large saucepan or Dutch oven over medium heat.
Add the chopped onion and diced sweet potato to the pan and cook until the onion softens and appears translucent, or about 5 minutes.
Add the minced garlic and cook for 1 to 2 minutes longer.
Stir in the thawed and squeezed chopped kale, the lentils, bay leaf, thyme, cayenne pepper, and the vegetable broth or chicken broth. Bring to a boil. Reduce the heat to low, cover the pan, and simmer the lentils for about 20 to 25 minutes. Check after 20 minutes. The lentils should be tender but not mushy.
Taste and add salt, as needed.