Cut tempeh block in two. Add to a medium saucepan and cover with water and bring to a boil. Parboil for 5 minutes as you prepare the teriyaki sauce. This serves to reduce the bitterness inherent in most store-bought tempehs.

In a medium sized bowl, whisk water, brown sugar, mirin, soy sauce, garlic and ginger powders until well combined. In a separate small bowl, whisk cornstarch with cold water and then add into the bowl with the other sauce ingredients and whisk until very thoroughly combined.

Remove tempeh from the stove and drain. Slice or chop into whatever shapes you desire.

Add oil to a large frypan and set to medium-high heat. Add sliced tempeh and brown for 3 minutes per side, splashing with a little soy sauce halfway to deglaze.

Pour teriyaki sauce into the pan and reduce heat to medium-low. Stir to coat and let sauce thicken as it cooks, about 3 minutes.