In a small mixing bowl, whisk the mayo, sweetened condensed milk, and lime juice until well combined. Set the sauce aside.
In a medium-sized mixing bowl, whisk together the salt, flour, cornstarch, baking powder, and potato starch to make the batter. Add in the egg and water and whisk until you form a batter. Add the shrimp to the batter and toss to combine. Set aside.
Add about 4 to 5 cups of oil to a deep frying pan set over medium-high heat.
Drop 1 teaspoon of batter into the oil. If it sizzles right away, it's ready for frying. Carefully lower 4 to 5 battered shrimp into the hot oil. Cook for 1 to 2 minutes on each side until golden brown. Remove from oil with a slotted spoon, strain, and set them on a wire rack to cool. Repeat with the rest of the shrimp.
Meanwhile, add ¾ cup vegetable oil to a pot or skillet set over low heat. Add the walnuts and let them fry for 1 to 2 minutes, constantly stirring to avoid burning. Remove them from the heat and let them cool for a couple of minutes.
In a small bowl, toss the fried walnuts with honey. Place them back in the oil and cook for another minute or until slightly crispy. Strain them and set them aside to cool.
Dip a fried shrimp into the mayo sauce and do a taste test. Adjust the sauce's flavor with more mayo, sweetened condensed milk, or lime juice to taste. Once ready, toss all the shrimp with the sauce until completely coated. Add the walnuts and stir to combine.
Arrange the shrimp over a bed of cooked rice, sprinkle with black sesame seeds, and enjoy.
Notes
Shrimp: Use raw, thawed, cleaned, and deveined shrimp. You can also use fish fillets; slice white fish, like Halibut or Cod, into 2-inch cubes and use that instead of shrimp.
Walnuts: If you don't have walnuts or are allergic to walnuts, skip them altogether or use a different nut.
Fry the shrimp in a separate pot from the walnuts because the honey will burn and stick to the shrimp. Or use the same pot, but wash it before using it for the shrimp.
Mayonnaise: For a more authentic recipe, I suggest using Kewpie, which is Japanese mayonnaise. It's thicker, tangier, and sweeter than its American counterpart. Add a little heat by mixing a bit of Sriracha to the sauce.