Melt the butter in a large skillet over medium high heat

Add the mushrooms and minced garlic. Sauté for 20 - 25 minutes, until the mushrooms cook down and the liquid evaporates completely. If there is excess liquid in the middle of cook time you can use a spoon to drain this off. There should be no liquid at the end

While the mushrooms are cooking, make the teriyaki sauce. Add the soy sauce, orange juice, brown sugar, and minced fresh ginger to a sauce pan. 

Heat over medium for around 5 minutes, stirring until the brown sugar dissolves

In a small bowl, mix together the cornstarch and water until dissolved

Add the cornstarch/water slurry to the remaining teriyaki sauce ingredients in the sauce pan, stir constantly for 3-5 minutes until thickened

Once the mushrooms have finished cooking and no liquid remains, stir in the teriyaki sauce and serve immediately