Add pork sausage to a skillet over medium-high heat. Brown the sausage and break it up into pieces with your spatula or wooden spoon. Add the onion and cook until just softened (you don't need to cook fully).
1/2 pound hot pork sausage, 1 onion
Remove from heat and add sausage and onions to your slow cooker.
Add the sweet potatoes, celery, lentils, and chicken stock to the slow cooker. Stir to mix.