If the nori sheets are already toasted and you are pressed for time, you can skip the toasting part, but even toasted nori can be perked up with a little heat. Toast each nori sheet over a gas burner on low, by slowly fanning it back and forth until the color turns bright green.No gas stovetop? You can also toast the nori in the oven. Turn on the broiler with the rack about 6 inches from the heating element. Place two pieces of nori on a baking sheet and pop them in the oven for only 10 seconds or until they turn green and smell toasty.
Cut each piece of nori in half width-wise so you have 16 pieces of toasted nori.
In a large bowl, combine the crab meat, scallions, mayonnaise, red pepper flakes (if using), and lime juice. Season with salt and pepper to taste, and mix well.
To assemble each roll, hold a piece of nori shiny-side down, and scoop 2 tablespoons of the crab mixture onto the left side of the rectangle. The filling should be at a diagonal, running from the top left corner to the bottom center of the nori.
Top the crab with a slice of avocado, some cucumber, and sprouts. Fold the bottom left corner of the nori over the filling before wrapping the long part of the nori around the crab and vegetables to form a cone.
Sprinkle with toasted sesame seeds and erve immediately—don’t let the nori get soft!