Pulse the shrimp in a food processor until it has a texture resembling ground beef. Add the celery, onion, water chestnuts, oyster sauce, garlic powder, sugar, white pepper, magic powder, and rice flour and pulse until evenly combined, being cautious not to overprocess the mixture to smooth (texture and chunks are a good thing).
Evenly spread 1/2 cup (60 g) of the shrimp mixture on one side of a slice of bread, about 1/4-inch (6 mm) thick. Repeat with the remaining slices and refrigerate for 15 minutes to set.
Meanwhile, preheat the oven to 350°F (175°C).
Add enough oil to a large skillet to coat the bottom and heat over medium heat. Gently place the toast mixture-side down into the oil (work in batches if necessary) and cook until lightly golden underneath, about 1 minute. Using a flat spatula, carefully flip the toast and cook until golden brown and crispy on the other side, about another 30 seconds.
Remove the toast slices from the skillet and transfer to a baking pan, shrimp sides up. Bake until cooked through and crisp on top, 1 to 2 minutes.
To serve: Transfer to a platter and serve warm topped with sesame seeds and sides of sweet and sour sauce and cucumber achaar.