Croquettes with Serrano Ham and Manchego Cheese

INGREDIENTS

• 2 tablespoons unsalted butter
• 2 tablespoons olive oil
• 1/4 cup finely chopped onion
• Coarse salt and freshly ground pepper
• 7 tablespoons all-purpose flour, plus more for shaping
• 3/4 cup whole milk
• 6 tablespoons finely chopped serrano ham
• 1/3 cup plus 1/4 cup grated Manchego cheese (about 1 3/4 ounces)
• 3 large eggs
• 1 1/2 cups fresh breadcrumbs
• Vegetable oil, for frying
• Fresh flat-leaf parsley sprigs, for garnish