Heat oven to 375 degrees. Grease a 9-cup capacity, 8 inch square, 4 inch deep, casserole or souffle dish; set aside. Place oil in a large non-stick skillet over medium high heat. When hot, add the onion and the mushrooms. Cook, stirring often, until heated through, about 4 minutes. Stir in the garlic. Cook 1 minute. Add the chives, orzo, cheese, broth, salt and pepper to taste. Mix well. Transfer to baking dish. Sprinkle the crumbs over top. (This can be prepared to this point several hours ahead and kept at room temperature.) Bake, uncovered, until the crumbs are browned, about 45 minutes. Serve hot, warm, or at room temperature.