Preheat oven to 350 degrees F (175 degrees C).

Remove stems from mushrooms and clean the caps thoroughly.

In a skillet, cook sausage and onion over medium heat until the sausage is browned and the onion is softened.

Drain any excess grease from the sausage and onion mixture.

In a bowl, combine the cooked sausage and onion mixture with cream cheese.

Mix thoroughly until all ingredients are well combined.

Place a large teaspoon of the sausage and cream cheese mixture on top of each mushroom cap, filling it generously.

Arrange the stuffed mushrooms on a baking sheet.

Bake in the preheated oven for 20-25 minutes, or until the mushrooms are tender and the filling is heated through and lightly browned.

Let cool slightly before serving.