1. Cook the potatoes in a pot with boiling water for about 12 to 15 minutes. Drain and return the potatoes back to the pot, so the residual heat will dry them slightly. Mash the potatoes until you get a puree.
2. Now add the ricotta, and season with salt and oil. Let cool.
3. Dry the shrimp and roughly chop.
4. Sprinkle a pan with oil and stirfry the shrimps with the crab meat and the chopped scallions for 5 minutes on high heat. Add to the potato puree.
5. With some flour in your hands, roll the potato mixture into balls (about 1-2 tablespoons per ball). Flatten the balls slightly when your palm to form the the croquettes. You should have about 8-10 croquettes.
6. Set up three shallow bowls: pour the flour in one; in the second, beat the egg with a pinch of salt. And in the third, combine 2 cups of bread crumbs with the dried herbs (oregano or thyme).
7. Dip one of your potato croquettes into the bowl of flour and cover completely, shaking off the excess. Then dip into the beaten egg. Finally, dip the egged croquette into the breadcrumbs and cover completely, shaking off the excess. Repeat with remaining croquettes.
8. Place the croquettes on a oven dish lined with foil and put them in a pre-heated oven at 374ºF for 20 minutes or until golden brown.
9. Let them rest for 3 or 4 minutes and serve it with lemon wedges.