Preheat oven to 350°.
In a small sauce pan heat the butter on high heat and sauté the diced onion until they are translucent – 2-3 minutes.
Add the minced garlic to the sauce pan and sauté till everything starts looking a little golden, 1-2 minutes. Be careful not to burn your garlic.
Pour sautéed onion in garlic into a bowl and set aside.
In the bowl of a mixer add cream cheese and milk and beat till fluffy.
Add to the mixer the mozzarella and Parmesan cheese and mix until combined.
Add in the sauteed onion and garlic, and mix.
Add cooked & crumbled bacon, salt & pepper to taste.
Next take your mushrooms and remove the stems.
Using a large spoon stuff the mushrooms with the cheese filling
Place the stuffed mushrooms in a buttered baking dish.
Place baking dish/dishes in your preheated oven and baked for 45 minutes or until the cheese is lightly golden brown and bubbly.