How to Make It
Combine shrimp, egg, and salt in a food processor; process until smooth, about 15 seconds, stopping to scrape down sides as needed. Add butter and 2 teaspoons sesame seeds; pulse until smooth with flecks of butter remaining, about 7 times. Spread 1/4 cup shrimp mousse on each bread slice.
Pour oil into a large nonstick skillet to a depth of 1/8 inch; heat over medium until shimmering, about 3 minutes and 30 seconds. Working in batches, add bread slices, mousse side down; cook until lightly browned, about 2 minutes. Flip toasts; cook until browned and mousse is firm to the touch, about 1 minute. Transfer to paper towels to drain.
Stir together chopped scallion, soy sauce, vinegar, sambal oelek, and remaining 1 1/2 teaspoons sesame seeds in a small bowl. Cut each toast into 8 triangles; transfer to a platter or board. Serve soy sauce mixture with toasts. Garnish toasts with sliced scallion, and serve.