Cook onion and bell pepper in butter over medium heat until very soft; do not brown. Remove from heat and stir in flour; when well blended, gradually whisk in milk or stock. Return to heat and cook until thickened, stirring constantly. Season with salt and pepper, and stir in Lea & Perrins and lemon juice. Stir in chicken, taste, and adjust seasoning. Pour into a shallow pan and chill until set.

Divide chicken mixture into 6 to 8 equal pieces and mold by hand into oval balls. Roll in beaten egg mixture then roll in bread or cracker crumbs. Chill until ready to fry.

To finish, pour oil to a depth of 3 inches in a deep saucepan set over high heat; heat oil to 390F. Working in batches, carefully lower the croquettes into the oil and fry, turning occasionally, until golden brown on all sides. Transfer to a wire rack set over a baking sheet to drain; return oil to temperature between batches.

Variations: Substitute turkey, crabmeat, or ham for the chicken. Form the croquettes around a tasty filling: finely diced ham, swiss, and mustard for a riff on chicken cordon bleu; creamed spinach; pepper-Jack cheese; etc.