Place your cubed tempeh into a steamer basket over boiling water. Cover and steam for 10 minutes. This softens the protein fibers and removes any bitter notes. Pat thoroughly dry afterward.
In a small bowl, whisk together the tamari, toasted sesame oil, peanut butter, minced garlic, grated ginger, water, and optional sriracha until completely smooth.
Heat the avocado oil in a large skillet over medium-high heat. Add the dried tempeh cubes. Fry, turning occasionally, until they form a golden-brown, crispy crust on most sides. If using mushrooms, toss them in for the last 2 minutes.
Turn the heat down to medium. Add the chopped kale directly to the pan. Pour the sauce evenly over everything. Toss continuously for 2 to 3 minutes until the kale wilts down into a vibrant green and the sauce thickens to coat the tempeh beautifully.
