How to Make It
Heat the oil in a large pot or Dutch oven over medium-high heat. Add the sausage and cook, breaking it up with a spoon, until browned, 8 to 10 minutes. Add the onions and cook, tossing occasionally, until beginning to soften, 3 to 5 minutes. Add the garlic and cook until fragrant, about 1 minute more.
Add the broth, mustard greens, sweet potatoes, lentils, ½ teaspoon salt, and ¼ teaspoon pepper to the pot. Bring to a boil, reduce heat, and simmer, partially covered, stirring occasionally, until the lentils and sweet potatoes are tender, 30 to 35 minutes.