PREHEAT oven to 375° F. Grease 13 x 9-inch baking dish.
HEAT oil in large saucepan over medium heat. Add onions; cook until softened. Whisk in flour; cook, stirring constantly, until mixture turns light brown, about 3 minutes. Gradually whisk in evaporated milk, garlic powder, salt and pepper; cook, stirring constantly, until sauce is thickened, about 5 minutes. Add 3 1/2 cups cheese; stir until melted. Add pasta and sausage; stir until thoroughly coated. Pour mixture into prepared baking dish. Sprinkle with crackers and remaining 1/2 cup cheese.
BAKE for 20 to 25 minutes or until bubbling around edges and golden brown.