Cook onion, bell pepper, and celery in 1 tablespoon butter in a 10-inch nonstick skillet over moderately low heat, stirring occasionally, until vegetables are softened, about 8 minutes.

Whisk together egg, sour cream, mustard, Worcestershire sauce, cayenne, and salt in a large bowl, then stir in scallion, cooked vegetables, and 1/4 cup saltine crumbs. Gently stir in crabmeat, then form into 4 cakes (2 1/2 to 3 inches in diameter). Dredge cakes in remaining saltine crumbs.

Heat oil and remaining 2 tablespoons butter in cleaned skillet over moderate heat until foam subsides, then cook crab cakes, turning once, until golden brown, 8 minutes total.