Melt the butter in a large skillet over medium high heat.
Add the mushrooms and minced garlic to a skillet. Sauté the mushrooms for 20-25 minutes, until they 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 the mixture 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, then 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.