In a large bowl, stir together shrimp, scallions, celery and sweet pepper.
In a small bowl, combine sour cream, mayonnaise, egg, lemon juice and spicy mustard.
Gently mix sour cream mixture into shrimp mixture.
Add breadcrumbs and stir to combine. Check for consistency - mixture should hold together easily enough to form shrimp cakes.
Season with salt and pepper to taste.
Add additional mustard or lemon juice if needed.
Tear off two 18-inch sheets of parchment paper, and place one on top of the other.
Using an ice cream scoop (or measuring cup), scoop out ½ cup portions onto the parchment. You should have about 8 portions.
Cover the top of the portions with a large sheet of plastic wrap. Using a small plate, press down each portion, forming a cake about 4 inches across.
Place cakes into the refrigerator for at least 30 minutes to firm up.
When ready to cook: Heat a nonstick skillet with about 1 tablespoon of oil over medium heat until hot. Working in batches, fit as many cakes as you can without crowding the pan. Be patient – let them sizzle away until you can see a brown crust forming on the bottom of each cake.
When ready to flip, use a thin spatula in one hand, and another spatula or similar implement in the other to help ensure the cakes don’t fall apart. They still might, but just do the best you can and smoosh them back together again. They will set up as they finish cooking.
Allow cakes to set up in the pan for about 5 minutes before serving.