Heat butter in medium saucepan over medium-low heat until melted and foaming. Add onions to pot; cook, stirring occasionally, until translucent, about 10 minutes Using whisk, add flour; cook, whisking occasionally, until well blended, about 2 minutes. Increase heat to medium high. Gradually add heated milk, whisking until smooth. Bring milk mixture to simmer. Simmer, whisking constantly, until thickened, about 4 minutes. Whisk in chorizo and cook 1 minute more. Remove pot from heat and whisk in Manchego, Adobo and parsley. Transfer milk mixture to bowl; chill until completely cold and set, at least 1 hour.
Scoop chorizo filling into hands; roll into log shape that measures about 2” long and 1” thick. Repeat with remaining filling to make 26 log shapes.
Place flour, eggs and breadcrumbs in separate dishes. Toss chorizo logs in flour, tapping off excess; dip in egg, then roll in breadcrumbs, pressing to adhere; set on baking tray.
Heat 1½” oil in medium, deep, heavy-bottomed pot over medium-high heat until hot but not smoking (350°F). Fry croquettes in batches, taking care not to crowd pan, until golden brown and heated through, about 2 minutes per batch. Transfer croquettes to paper towel-lined tray to drain. Serve warm.
Are you a seafood person? Or perhaps you like poultry or pork. No problem! You can easily convert these chorizo croquettes into croquettes with different fillings. Simply replace the chorizo with an equal amount of leftover chopped chicken, smoked ham, shrimp or reconstituted salted cod (bacalao).