Place the rice in a rice cooker with 4 cups water, or cook on the stove according to package instructions. Cook the rice while you prepare the rest of the meal. Start cooking the stir fry when there is roughly 20 minutes left on the rice. (You can also cook the rice ahead of time & reheat before serving.)
In a small bowl, whisk together the sauce ingredients and set aside.
Add the peanut oil to a large nonstick skillet or wok & warm over high heat until shimmering.
Add the mushrooms to the pan & cook, stirring occasionally, until golden brown, about 5-10 minutes, depending on the heat of your stove. Remove the mushrooms from the pan and set aside on a plate.
Without adding anymore oil to the pan, add the green beans and let cook on high heat, stirring occasionally, until blistered in spots, roughly 3-5 minutes. Remove the green beans from the pan and set aside on a plate.
Add the beef to the pan, using a spatula to break up into small pieces while cooking. Stir fry until nicely browned, 3-5 minutes.
Add the green onion and garlic to the pan and cook for another 1 or 2 minutes until fragrant.
Add the sauce to the pan and stir to coat. Then add the mushrooms and green beans and stir to coat. Cook for a minute or so to warm everything thoroughly, and serve over the cooked short grain brown rice. Top with extra sriracha if you like things spicy!