Preheat olive oil in a large pot over medium heat. Add diced onions and cook for 5 minutes, until starting to get translucent. Add garlic and cook for 1-2 minutes, stirring.
Add ground sausage to the pot, breaking down the sausage into smaller pieces with a spatula or spoon. Cook until browned, about 10 minutes.
Pour lentils into the pot. Stir to coat.
Add in chicken broth. You may not want to add all of it right away. Start with 1.5 quarts and then go from there getting it to your desired consistency. You can also add more as it is cooking.
Stir in sweet potatoes, salt and pepper. Bring soup to a boil and then cover with a lid and simmer for 20-30 minutes, to get the flavors to meld. Check to make sure the sweet potatoes are soft.
Remove lid and stir in the kale. Stir until the kale wilts.