In a large skillet, saute rice in butter until lightly browned. Add green onions and garlic; cook and stir until tender. Stir in mushrooms., Transfer to a 1-1/2-qt. slow cooker. In a small bowl, whisk water and beef base; pour over rice mixture. Cover and cook on low for 3 to 3-1/2 hours or until rice is tender and liquid is absorbed. Fluff with a fork. If desired, add sliced green onions.