Combine ham and mustard in a food processor and pulse until it forms a smooth paste.
Melt the butter in a heavy saucepan over medium-low heat. Add the shallots and saute until translucent. Add 2 tablespoons of the flour and cook, stirring constantly until well blended but not browned, about 2 minutes. In the meantime, gently heat the milk to a gentle simmer but do not let it boil. Gradually stir the milk in to the saucepan, whisking constantly to eliminate clumps. Stir until the sauce has thickened and the whisk leaves trace marks in the surface, about 5 minutes. Stir in cheese and season with salt, pepper, nutmeg and pimenton. Remove from heat. Add the pureed ham and mix until well combined.
Bring to room temperature. Pour the mixture in a shallow bowl or lined baking sheet. Cover with plastic wrap and refrigerate until set, at least 1 hour.
Lay out the remaining 1/2 cup of flour, beaten eggs, and breadcrumbs in separate mixing bowls. Scoop out 1 heaping tablespoon of the bechamel mixture and roll each croqueta in the flour to coat, shaking off the excess flour. Dip into the egg mixture with a fork or slotted spoon, allowing excess to drip off, and then roll in breadcrumbs. They should be well coated so the filling doesn't leak when cooked.
Add about 3-inches of oil to a large, deep pot. Heat over medium-high heat to 365 degrees F. Working in batches, carefully add the croquetas a few at a time. Do not crowd them or the temperature of the oil will drop. Gently turn until brown on all sides, about 2 minutes. Remove with a slotted spoon and drain on paper towels or re-purposed grocery paper bags. Bring the oil back up to the correct temperature in between batches. Serve immediately.
Makes 20 to 24 croquetas