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1
Preheat oven to 350 degrees F (175 degrees C).
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2
Lightly grease a 2-quart casserole.
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3
Place carrots into a large pot and cover with salted water; bring to a boil.
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4
Reduce heat to medium-low and simmer until tender, about 4 to 6 minutes.
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5
Drain and transfer to a large bowl.
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6
Melt 1/3 cup butter in a heavy saucepan over low heat; cook and stir onion and garlic in the melted butter until tender, 5 to 10 minutes.
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7
Season with salt.
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8
Whisk flour into onion mixture until smooth; cook, stirring constantly, until flour mixture is a paste-like consistency and light brown, 5 to 10 minutes.
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9
Increase heat to medium; gradually stream milk into flour mixture until thick and bubbly, 5 to 10 minutes.
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10
Whisk Cheddar cheese, about 2 tablespoons at a time, stirring until cheese melts each time before adding more, forming a smooth sauce, 10 to 15 minutes.
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11
Stir in mustard powder, white pepper, celery seed, and 1/2 teaspoon salt until incorporated, 1 to 2 minutes.
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12
Pour sauce over carrots and toss to coat; transfer to the prepared casserole dish.
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13
Mix 2 tablespoons melted butter and bread crumbs together in a bowl until well coated; sprinkle over carrot mixture.
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14
Bake in the preheated oven until sauce is bubbling and topping is browned, 25 to 30 minutes.
