Whether you use frozen or fresh shrimp and squid, make sure they are dry before frying. You can thaw frozen ones and blot them with paper towels. You can clean fresh squid and slice them into rings about 1/4 inch thick. Alternatively, you can get them pre-cleaned and cut from a fishmonger. You can fry the small tentacles along with the rings.
To make the squid more tender, juicy and tasty, soak the rings in milk. Put them in a big bowl and cover with milk. Keep them in the fridge for at least two hours or as long as overnight.
Whisk the flour, paprika, pepper, and salt in a shallow dish. Pat the squid rings dry and coat them with the flour mixture, shaking off the excess. Do the same with the shrimp.
In a large skillet, heat oil to a depth of about 1/2 inch over medium-high heat. Working in batches, fry the squid rings for about 2 minutes per side, or until they are golden and crisp. Transfer to paper towels to drain and season with more salt if desired.
To make the sandwiches, you can toast the bread rolls after slicing them in half, or leave them as they are. Then, spread some mayonnaise or garlic sauce on both sides of each roll. Add the crispy squid rings to the rolls and squeeze some lemon juice over them. Enjoy your delicious sandwiches.