Combine flour, mustard, salt, and pepper; dredge brains in flour mixture, coating well. Set aside.
Sauté onion and green pepper in hot oil in a large Dutch oven until tender. Add brains, and cook until browned, turning gently to brown all sides.
Gradually add 1/2 cup half-and-half. Cover and cook over low heat 40 minutes. Combine remaining half-and-half, Parmesan cheese, and sherry. Pour over brains mixture; cook an additional 5 minutes over low heat. Serve immediately over toast points, if desired.