In a crusted manner; with a crust or crusty surface; in the manner of something that is covered with a crust.
From crusted (adjective, meaning having a crust) plus the adverbial suffix -ly meaning 'in the manner of.' It transforms the adjective into an adverb describing how something is done.
Old bread that's been left out sits crustedly on the counter—the water inside gradually migrates to the surface and evaporates, hardening the outside while the inside actually becomes stale (a different process from the crust formation).
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