Add olive oil to a medium skillet. Heat over medium-high heat.
Add the sliced mushrooms and onion in the skillet.
Saute for two minutes without stirring. Stir well.
Saute for an additional six-seven minutes.
Whisk the vinegar, water, and brown sugar together in a small bowl. Set aside.
Add the garlic, salt, pepper, and crushed red pepper in the skillet. Stir to combine.
Saute on medium for an additional minute. Reduce heat to low.
Spoon the sauce over the contents in the skillet. Stir to combine.
Continue to saute for one more minute. Serve.
Enjoy!