Step 1Heat the oven to 375 degrees. Grease a 9-cup capacity, 8-inch-square, 4-inch deep, casserole or souffle dish; set aside.
Step 2Place the oil in a large nonstick skillet over medium-high heat. When hot, add the onion and 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 the baking dish. Sprinkle the crumbs over the top. (This can be prepared to this point several hours ahead and kept at room temperature.)
Step 3Bake, uncovered, until the crumbs are browned, about 45 minutes. Serve hot, warm or at room temperature.