Slice your beef into tiny cubes. I bought stir-fry beef and cut it into smaller pieces.
Mince the garlic cloves.
Find a large mixing bowl.
Add the minced garlic, balsamic vinegar, soy sauce, dijon mustard, olive oil, and red pepper flakes into the bowl and mix them up well.
Add in the beef and stir well, coating all sides. Marinate for 4 hours.
In a high rimmed pan, heat to medium and add some oil.
While that’s heating, slice the onions and mushrooms.
Cook the onion in the oil for about 5 minutes until nice and golden, and then add the sliced mushrooms. Season both with salt and pepper and cook for about 3 minutes, removing both once golden.
Keeping the heat at medium, add the beef with the sauce and cook for 10 minutes (stirring only on occasion).
Add back the onions and cook until tender. (I did about 6 more minutes, read further description above if needed, so the meat isn’t too tough.)
Serve with your favorite side. (We did mashed potatoes and roasted broccoli.)
Enjoy!