Slice off and discard dark-green top and root of leek. Cut in half lengthwise, and rinse under cold water to remove any grit; thinly slice.
In a large, deep skillet or saucepan, heat oil over medium heat. Add leek and garlic and cook, stirring constantly, for 3-4 minutes or until leek begins to soften.
Add mushrooms and cook, stirring frequently, for 2-3 minutes. Stir in tomatoes, breaking up with a spoon, add water and orzo.
Bring to a boil and cook, stirring occasionally for 12-15 minutes or until orzo is tender and mixture has thickened. Remove from heat and stir in basil. Pass cheese to sprinkle on top if desired. Add salt and pepper to taste.