Preheat your oven to 350°F. Lightly grease a 2-quart baking dish.
Heat the olive oil in a large, deep skillet over medium-high heat for about 2 minutes.
Add the onion, celery, bell pepper, mushrooms, kosher salt, and black pepper. Cook, stirring often, until the vegetables are tender-crisp, for about 5 minutes.
Add the minced garlic, thyme, sage, and pork sausage. Cook, stirring and breaking up the meat as you cook, until the meat is no longer pink, for about 5 more minutes.
Transfer the skillet contents, undrained, to the prepared baking dish.
Bake the stuffing uncovered until browned, for about 30 minutes.
Allow the stuffing to rest for 5-10 minutes before serving.
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Notes
You can use pre-packaged sausage meat or make your own. You don't have to use pork - beef, chicken, or even turkey sausage meat all work. Here's a recipe for sausage patties that will give you an idea of what spices to use and their measurements.
I like Cremini mushrooms, but white mushrooms work, too, as do portobello mushrooms.
You can use any color of bell pepper. Red adds a nice splash of color to the dish.
You can make this dish a bit spicy by adding a pinch of cayenne when you add the other spices. A pinch of cumin is not a traditional stuffing seasoning, but it adds warmth and is delicious.
There's no need to add eggs. Eggs are used as a binder in bread stuffing. In sausage stuffing, they're not needed.
You can keep the leftovers in the fridge, in an airtight container, for 3-4 days. Reheat them in the microwave or in a 350°F oven. This is one of those recipes where the leftovers taste just as good as the freshly made dish.