Bring water and rice to a boil in a small pot. Reduce heat, cover, and simmer until water is absorbed, about 15 minutes. Remove from heat, stir in 1 tablespoon oil, and season with salt and pepper. Cover, and let stand.
Meanwhile, heat remaining 4 tablespoons oil in a large saute pan over medium-high heat. Add shiitakes in a single layer, and cook, stirring often, until browned and crisp, about 3 minutes. Reduce heat to medium, and cook for 2 minutes more. Add garlic, and cook until light gold, about 2 minutes. Stir in vinegar, and season with salt and pepper. Transfer rice to a platter, top with shiitake mixture, and sprinkle with scallions. Serve immediately.