Method

1. Preheat oven to 350°F. Grease a 9″x13″ baking dish.
2. In a large pot bring water to a boil. Add carrots and cook for 5-7 minutes, until carrots have soften slightly. Drain carrots; set aside. (Note: depending on the size of your pot, this may need to be done in 2 batches).
3. In a medium pot, over medium-low heat melt butter. Add onions, sauté until softened.
4. Add flour to butter and onions; combine and allow to cook 1 minute while stirring.
5. While whisking, add 1 cup of milk; mix until combined. Add remaining milk, 1 cup at a time, mixing to combine each time.
6. Add salt, pepper, celery salt and mustard; whisk to combine. Bring sauce to a low boil and continue to cook until it thickens.
7. To the prepared baking dish, add half the cooked carrots. Cover evenly with 1 cup of shredded cheese and half the cream sauce. Repeat with remaining carrots, cheese and sauce.
8. Add bread crumbs to 1/4 cup of melted butter and mix thoroughly. Spread mixture evenly over the casserole.
9. Bake uncovered 25-30 minutes, until browned and heated through. Serve immediately or chill and reheat to serve.