Combine all of the "for the sauce" ingredients in a bowl and whisk together. Set aside.
Heat 2 tbsp. avocado oil over medium-high heat. When hot, but not smoking, add in the green beans and cook, stirring often, until tender and browned, about 7 minutes. While your green beans are sauteeing, season them with a little salt and pepper, to taste.
Add in the shiitake mushrooms, the minced garlic, and the grated ginger and reduce heat to medium. Continue to saute for about 2 more minutes.
Pour in the sauce and cook, stirring occasionally, until the sauce has thickened and reduced, about 3 more minutes.