"Serrano ham is part of the Spanish diet and is as common as bacon, if not more. Homemade ham croquettes are easy to make once you get the hang of making the roux and shaping them. Vary flavors and color by substituting for the ham with canned tuna, baked salmon, cooked spinach, or even mashed sweet potato. Croquettes can be frozen after breading and before frying."
Melt olive oil and butter in a large nonstick pot over medium heat. Add onions and stir with a wooden spatula until translucent, about 3 minutes. Add flour and stir to create a thick paste, about 2 minutes. Pour in milk while stirring; it will seem lumpy at first, but it will smooth out to the consistency of mashed potatoes, about 5 minutes. Add nutmeg; keep stirring until a dough forms and starts to pull away from the sides of the pot, about 5 minutes more.
Stir ham into the pot and mix into the dough until well combined, about 2 minutes. Add parsley. Transfer dough into a shallow bowl and allow it to cool.
Place eggs in a bowl and beat. Place bread crumbs in a separate bowl.
Oil the palm of 1 hand; scoop 2 tablespoons of dough into the oiled hand using a silicone spatula. Shape the dough into an oblong capsule. Place the croquette into the bowl with the eggs. Use a fork to transfer it to the bowl with the bread crumbs; roll using fingers to coat well. Transfer to a plate; repeat with remaining dough, placing croquettes side-by-side.
Heat 1 1/2 inches oil in a large skillet over medium heat. Add croquettes to the skillet without crowding and brown on all sides, 6 to 8 minutes. Place on a paper towel to absorb excess oil.