Heat olive oil with butter in a medium skillet over MED-LOW to MED heat until butter is melted. Add diced onions and cook until onions are translucent, about 5 minutes.
Add in the mushrooms and cook for approximately 5 more minutes.
Stir in the garlic until fragrant, about 1 minute.
Add in the remainder of the gravy ingredients and stir to blend.
THEN blend in the beurre manie as it melts the butter and distributes the flour within the paste.
Once there's evidence of thickening, add in the meatballs and simmer for about 5- 7 minutes.
Serve over mashed potatoes, eggs noodles or on the side with gravy spooned over them.