Make the croquettes In a large pot, melt the butter in a saucepan over moderately-high heat. Add the onions and sauté until soft. Stir in 1½ cups flour until combined. Gradually add milk and bring to a boil, reduce heat to low and allow mixture to thicken for about 5 min. Remove from the heat.
While the mixture is still hot, add cheese, ham, Sazón and Adobo, stir to combine. Transfer the mixture to a baking dish and place in the refrigerator for at least two hours.
Once chilled, shape the ham mixture into small logs, about 3/4-inch thick wide and three inches long. You should have about 16 logs. Using the remaining flour pass the croquettes through flour, dip in eggs and breadcrumbs.
Heat 1½" oil in medium, deep, heavy-bottomed pot over medium-high heat until oil registers 350°F. Fry croquettes in batches until golden and crisp, about 3-4 minutes per batch. Transfer to paper towel-lined plate to drain.
Make the tuna In a large bowl, whisk all of the ingredients together except for the tuna and adobo. Add the tuna season to taste with Adobo. Let the tuna sit in the marinade for at least 5 minutes or refrigerate until ready to use.
Make the sauce In a medium bowl, mix the sour cream with the Sazón, aji paste and sugar.
For serving, arrange the croquettes on a platter and top with the tuna poke. Finish by drizzling with the sauce. Serve right away.