Cajun Sausage & Cream Cheese Stuffed Mushrooms

ounces, white button mushrooms

pork sausage (regular or hot)

medium onion, finely diced

garlic, finely minced

cream cheese

grated Parmesan cheese

shredded cheddar cheese

Cajun seasoning

optional garnish: green onions or parsley (don't add until after the stuffed mushrooms are baked or the garnish will burn)