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).
Remove from heat add to your slow cooker.
Add the sweet potatoes, celery, lentils, and chicken stock to the slow cooker. Stir to mix.
Cook on high for 3-4 hours.
Stir in the spinach leaves and wilt them. Season with salt and pepper.
If your stew is thick and you want to thin it out, add a little more chicken stock to it until you get the desired consistency.
Store leftovers in an airtight container in the fridge for up to 1 week.