Cut salmon fillet into 4-ounce pieces. Arrange the salmon fillets on a baking sheet. Pat the salmon dry with paper towels.
Season the salmon with sea salt or Kosher salt and freshly cracked black peppercorns on both sides. Refrigerate the salmon until you are ready to cook it in the deep-fryer.
To make the beer batter, grab a medium-size mixing bowl and add the all-purpose flour, onion salt, garlic powder, onion powder, Old Bay seasoning, additional sea salt, and freshly cracked black peppercorns, and mix thoroughly.
In a small bowl, crack 1 egg and whisk with a fork, then add the beer. Add the egg-and-beer mixture to the flour mixture. Mix until combined, but do not overmix it. A few lumps in the batter is fine. Set the batter aside.
Fill the Masterbuilt Propane Fryer to the fill line with frying oil, follow the lighting instructions, and preheat the oil to 350 °F.
When the oil is hot, dredge the salmon in the batter and shake off any excess. Gently submerge the salmon into a basket in the hot oil. Shake the basket carefully to prevent the salmon from settling at the bottom. Also, do not overcrowd the basket. I placed 4-5 pieces into each basket.
Working in batches, fry the salmon for 4-5 minutes until golden brown. Insert an instant-read thermometer until the salmon reads 145°F.
After the salmon is cooked, remove the hot oil basket and shake it to let excess oil drip off.
Transfer the golden-brown, beer-battered salmon to the parchment-lined sheet pan using tongs or a large slotted spoon.
Finish the salmon with a pinch of sea salt.
If frying additional salmon, skim off any floating batter and discard.
Avocado Mayo Spread: In a bowl, add two ripe, peeled avocados, 1 1/2 cups mayo, 1/3 cup diced red onion, 2 tablespoons EVOO, and sea salt and freshly cracked black peppercorns to taste. Mix until combined. This will make about 1 1/2 cups of avocado mayo. You can store the rest in the fridge in an airtight container.
Spread the avocado spread on both sides of the toasted bun, place a salmon fillet on the bottom bun, and add your favorite toppings.
Crown with the top of the bun and serve.