Mix together all ingredients well.  Make sure not to overmix as it can make the ground chuck tough and stringy.  Make 2" diameter burger patties and flatten well.  These will shrink to fit the stuffer mushroom caps perfectly.  Cook a tiny sample patty and check for seasoning.  

Heat up a pan, or griddle on medium high heat, and add in some vegetable oil.  Sear the burgers on both sides, making sure to get nice, dark caramelization on the patties.  

If you would like a rarer hamburger, you can skip the oven, but if you want your hamburgers well done, transfer them to a baking tray and bake in a 375 degree oven for 5 minutes to finish them.  Serve with your favorite slice of cheese, or top with your favorite BBQ sauce.

Add some vegetable oil to a small pot and add in the thinly sliced red onions.  Cook on medium low heat.  This will allow the onions to cook nice and slow with out burning them.  This will take 20-25 minutes but will be well worth the wait.  The onions should be nice and dark brown.  When the onions are almost done, add in the balsamic, reduce, and finish with the maple syrup.  

Remove the stems of the mushrooms, scrub and rinse well.  Toss with vegetable oil, salt, pepper, and garlic powder and bake in a 375 degree oven for 5-10 minutes until the mushrooms are nice and soft, but not too over cooked.  Dry off well with paper towel and keep at room temperature to serve with the burgers.  

Build the burgers with the caramelized onions, lettuce, tomato, optional condiments, and use a wooden skewer to hold everything together.  Enjoy!