Preheat oven to 375°
Place the stemmed mushrooms on a parchment lined baking sheet. In a large skillet, sauté the onion and sausage over high heat until the sausage is cooked through, about 8 minutes.
While the sausage is cooking, puree the mayonnaise, chipotle pepper, and adobo sauce in a blender or food processor.
Once the sausage is cooked through, remove from the heat and stir in the chipotle mayo.
Spoon the filling into the mushroom caps, being careful not to overfill.
Bake 20-25 minutes, or until the stuffing starts to brown nicely along with the mushrooms.