Melt butter in a saucepan on medium-low heat. Slowly, whisk in milk and flour. Constantly stir until a thick sauce forms.
Add the cuban seasoning, red pepper flakes, 1/2 cup of shredded mozzarella, grated parmesan cheese, and tomato paste to the sauce. Mix it all together slowly. Let it simmer for 5 minutes on low heat.
Let the filling rest at room temperature then refrigerate for 2 hours. The mixture has to harden in order to form into logs and coat evenly. If this step is skipped, it will be very hard to form logs, coat with breading, and fry. This is an important step.
To make the croquettes, spoon approximately 1 tablespoon of the mixture and form a log 3 inch long and about 3/4 thick. Using the remaining shredded mozzarella, grab a pinch of mozzarella and add it into the middle of the log before you completely form it. This is where the melty cheesy goodness comes into play! Repeat until you have all the croquettes done.
Crack and whisk eggs in a bowl and place the breadcrumbs in a second bowl. Dip each log in the egg and then completely cover in breadcrumbs. Let them rest at room temperature for 5 minutes before frying.
In a large frying pan, add enough vegetable oil to cover the croquettes completely. Make sure the oil is at 375 degrees F (medium heat). Fry each one until golden brown, approximately 3-4 minutes each. Don’t crowd the pan. Gently transfer to a paper towel lined plate to drain. Serve hot.