Preheat the oven to 375 F.
In a large pan over medium heat, add the ground pork and sausage seasoning. Brown the meat for 5-10 minutes. Transfer the meat from the pan to a plate with a slotted spoon.
There should be about 2 Tbsp of fat left over in the pan. If not, add a bit more of your favorite fat. Add the onions and cook them down for about 10 minutes. Stirring occasionally.
While the onions are cooking, remove the stems from the mushrooms and discard half of them. Place the other half of the stems in a food processor and pulse until the stem bits are about half the size of a grain of rice.
Add the stems and sausage back to the pan and stir to combine.
With a small spoon, fill the mushroom caps with the sausage mixture and place on a baking sheet.
Bake for 20 minutes, or until the sausage has browned on top.
While the mushrooms bake, add the balsamic vinegar to a small sauce pan and bring to a boil. Reduce heat and simmer the sauce until it reduces by half, for 10-15 minutes. Stirring occasionally.
Remove the mushrooms from the oven and drizzle with balsamic reduction. Serve warm or at room temperature.