Whisk the ingredients for the sauce together until well combined. Set aside.
Using a large skillet, heat the oil over medium-high heat. Add the mushrooms. Allow to saute undisturbed for 1-2 minutes, and then begin to stir around the pan with a wooden spoon. Saute for another 2-3 minutes or until tender.
Pour in the sauce and stir to coat the mushrooms, bringing the sauce to a low simmer and allowing it to thick for 1-2 minutes.
Add in the spinach and sundried tomatoes and stir until the spinach is wilted.
Serve warm as a side dish topped with parsley.
Notes
All nutritional information are estimations and will vary. Estimations do not include optional ingredients.