Melt 2 tablespoons butter in a medium saucepan or sauté pan until foaming subsides. Add celery, scallions, and bell pepper, and sauté until just beginning to soften, about 2 minutes. Stir in flour and cook over medium heat for 1-2 minutes. Slowly whisk in milk and cook, stirring, until very thick and pulling away from sides and bottom of pan, about 2 minutes. Remove from heat and let stand 1 minute. Gradually whisk in egg yolk, followed by Dijon mustard, salt, and pepper. Let mixture cool for 5 minutes.

Fold crabmeat into the sauce, gently breaking up the lumps, but leaving some large pieces intact. Shape into 6 cakes (roughly, 1/3-cup each), and coat with breadcrumbs. Cover and refrigerate for at least 1 hour, and up to 3 hours.

Heat olive oil and remaining 1 tablespoon butter in a 12-inch sauté pan until foaming subsides. Cook crab cakes about 4 minutes per side until golden brown and heated throughout, using a thin spatula to gently flip. Serve hot with a squeeze of lemon juice and a dollop of remoulade, if desired.