In a small saucepan, heat milk over medium with the ham ends/trimmings (if using), then hold, covered, just below a simmer until ready to use.

In a 3-quart saucepan or saucier, melt butter with olive oil over medium heat until butter is melted. Add onion and cook, stirring, until very soft but not browned, about 8 minutes. Meanwhile, remove and discard ham trimmings from milk (if using).

Stir in minced jamón followed by ½ cup plus 2 tablespoons (80g) flour until flour has formed a paste with the fat that evenly coats all the solid ingredients. Continue to cook, stirring, until raw flour scent is gone, about 1 minute. Whisking constantly, add hot milk in a thin, steady stream, or in increments of a couple of tablespoons at a time, whisking thoroughly and getting into all corners of the pan to maintain a homogeneous texture. Continue until all milk has been added and mixture has thickened to a paste-like consistency. Remove from heat and season croqueta filling with salt and nutmeg.

Scrape filling into a small heat-proof baking dish, rimmed eighth sheet pan, or other vessel that can hold the mixture in an even layer. Cover with plastic and refrigerate until fully chilled and set, at least 6 hours and up to overnight.

When ready to form croquetas, line a rimmed baking sheet with parchment. Set three wide, shallow bowls on a work surface. Fill the first bowl with the remaining 1 cup flour, the second with beaten eggs, and the third with panko. Wetting hands as necessary to prevent sticking and working in batches as you go, form filling mixture into roughly 2-inch by 1-inch cylinders. Place them first in the flour, rolling until well coated, then shake off excess and transfer to eggs. Gently roll to coat evenly with egg wash, lifting each croqueta to allow excess egg to drain off, then transfer to panko. Gently roll and press panko onto croquetas until fully coated. Transfer breaded croquetas to prepared baking sheet and repeat with remaining filling until all croquetas are breaded (you should have about 24). Cover with plastic, transfer to the refrigerator, and chill for at least 2 and up to 24 hours.

When ready to fry, in a Dutch oven or wide pot, heat at least 1 inch of oil over medium-high heat until 340°F (170°C) on an instant-read or frying thermometer. Set a wire rack in a rimmed baking sheet and line with paper towels. Working in batches to avoid crowding the oil, fry croquetas until golden brown all over, 2 to 3 minutes. Using a spider skimmer, transfer to paper towels to drain, then repeat with remaining croquetas. Serve right away.