Select saute setting on a 6-qt. electric pressure cooker. Adjust for medium heat; add butter. When butter is hot, cook and stir rice until lightly browned, 3-5 minutes. Press cancel. Add mushrooms, green onions and garlic. In a small bowl, whisk water and beef base; pour over rice mixture.
Lock lid; close pressure-release valve. Adjust to pressure-cook on high for 4 minutes. Let pressure release naturally. If desired, serve with additional green onions.