Bread the croquettes generously; this will prevent them from bursting open when frying.
- 3 Tbsp. butter
- 1 1⁄4 cups flour
- 3 1⁄4 cups milk, hot
- 1 small yellow onion, chopped
- 1 sprig flat-leaf parsley
- 1⁄3 lb. sliced serrano ham, chopped
- 1⁄3 lb. medium shrimp, peeled, deveined, and finely chopped
- Salt, to taste
- Spanish olive oil
- 3 cups dried fine bread crumbs
- 3 eggs, lightly beaten
Heat butter in a saucepan over medium heat. Add 1⁄4 cup of the flour; cook until just golden, about 3 minutes. Whisk in milk, onions, and parsley; bring to a boil over medium-high heat. Reduce heat to medium; simmer, stirring frequently, until thick, 18–20 minutes. Strain sauce through a fine sieve. Return to saucepan with ham and shrimp; bring to a boil over medium-high heat. Reduce heat to medium-low; simmer until thick, 12–14 minutes. Season with salt. Chill overnight.
Pour olive oil into a large pot to a depth of 2". Heat until a thermometer registers 375°. Put remaining 1 cup flour, bread crumbs, and eggs into three separate wide shallow dishes. Using flour-dusted hands, form 2 tbsp. chilled ham mixture into a 3"-long oval; dredge in flour, roll in eggs, then coat with bread crumbs. Shake off excess. Repeat to create another coating of eggs and bread crumbs (but not flour). Transfer to a plate. Repeat process to make 20 croquettes in all. Fry croquetas until golden, about 1 1⁄2 minutes. Drain on paper towels.