How to Make It
In a heatproof medium bowl, cover the shiitake caps with boiling water and let stand until softened, about 25 minutes. Drain well, then coarsely chop and transfer to a food processor. Add the parsley, shallot, garlic, lemon zest and lemon juice and pulse until the parsley is finely chopped.
Meanwhile, in another heatproof medium bowl, combine 1/2 cup plus 2 tablespoons of boiling water with the soy sauce and agave. Stir in the TVP and let stand until rehydrated and tender, about 15 minutes. Stir in the shiitake-parsley mixture, black beans, flax meal and salt. Using a fork, lightly mash half of the beans against the side of the bowl and stir to combine.
Using moistened hands, form the mixture into six 3/4-inch-thick patties; pack tight. Transfer to a parchment paper–lined baking sheet.
Light a grill or preheat a grill pan; oil the grate or the pan. Grill the burgers over moderately high heat until charred on the bottoms, about 5 minutes. Using a spatula, carefully flip the patties and cook until lightly charred and heated through, 3 to 5 minutes longer. Serve the burgers on the buns with the Banana Pepper Mustard, mayonnaise, lettuce and tomato.
Textured vegetable protein (TVP) is available at large supermarkets and specialty food shops; look for the Bob’s Red Mill brand.