In a large bowl, combine the crab, mayonnaise, wasabi, 3/4 of the chopped apples, all of the fennel (including green tops), the radishes, juice and zest of 1 Meyer lemon, and salt, to taste. Cover and refrigerate until ready to serve.
In a small bowl, combine the remaining chopped apples with remaining lemon juice and zest. Season with salt, to taste, and set aside for garnish.
Toast: Remove the crusts from the brioche and cut into 3/4-inch thick slices. Stack the slices and cut into 1 1/2-inch squares. Alternately, you could use a cookie cutter to make circles or other small shapes of your choice. You should have about 48 pieces when finished.
Working in batches, melt some of the butter in a large skillet over medium heat. Add the brioche, in batches, and toast until golden brown, turning over and adding more butter, if necessary. Repeat with remaining butter and brioche. Toasts can be made up to 48 hours in advance and stored in an airtight container at room temperature.
To serve, put a rounded tablespoon of crab salad on each brioche toast and top with a sprinkling of chopped lemon-marinated apples.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.