Preheat oven to 350 degrees F.
Arrange bread cubes on a baking tray and bake for 4-5 minutes until dried out and golden brown.
Cut off the tops of the peppers and remove seeds and white membrane to create cups. Test to see if pepper cups will sit upright. If not, trim the bottoms just barely so the peppers sit flat, being careful not to cut through to the center.
In a medium skillet over medium heat, cook the sausage, crumbling into small pieces as it cooks, until no pink remains. Transfer to a separate dish and drain off any excess grease.
In the same pan, add olive oil and heat over medium heat. Add fennel, celery, and onion and cook until soft. Add back in cooked sausage crumbles and stir to combine. Mix in sage, salt and pepper, and toasted bread cubes; toss to coat. Gradually add chicken stock, 1/4 cup at a time until absorbed. Stuffing should be moist, but not wet.
Fill peppers with stuffing. Arrange on a baking dish coated with nonstick cooking spray and bake for 15-20 minutes or until peppers are soft and the stuffing is lightly browned and crunchy. Allow to rest for 5 minutes before serving.
Recipe adapted from: https://skinnyms.com.