How to Make It
Preheat the oven to 375°. Spread the beet pulp evenly on a parchment paper–lined rimmed baking sheet. Spread the carrot pulp evenly on a second parchment-lined baking sheet. Roast for about 30 minutes, stirring occasionally, until the pulp is dry and browned in spots. Scrape into a bowl and let cool completely. Leave the oven on.
On the rimmed baking sheet, toss the mushrooms with the kidney beans, tofu and canola oil. Spread the mixture in an even layer and roast for 30 minutes, stirring occasionally, until browned and dry.
In a food processor, pulse the spelt until finely chopped; scrape into a large bowl. Add the mushroom mixture to the processor and pulse until finely chopped; add to the bowl. Add the roasted beet and carrot pulp, the celery pulp and all of the remaining burger ingredients and mix well. Pack the mixture into four 1/2-inch-thick burgers.
Heat a grill pan. Spread the butter on the buns and toast cut side down over moderate heat until golden, about 2 minutes. Transfer the buns to a plate.
Brush the grill pan with oil and heat. Add the burgers and cook over moderate heat, turning once, until nicely charred, about 6 minutes. Top each burger with a slice of cheese, cover and cook until the cheese is melted, about 2 minutes longer. Set the burgers on the buns. Top with lettuce and tomato, spread mustard on the top buns and serve.