Recipe: Mushroom Burger With Pickled Maitake Aioli

Ingredients

• For the Pickled Maitake Aioli
• 8 ounces maitake mushrooms, cut into small pieces
• 1¼ cups white vinegar
• ⅓ cup sugar
• ¼ teaspoon kosher salt, plus more to taste
• 1 egg yolk
• 1 tablespoon fresh lemon juice
• 1 cup canola oil
• 3 tablespoons heavy cream
• ¼ teaspoon minced garlic
• For the Mushroom Burger
• ¼ cup quick-cooking oats
• 3 tablespoons heavy cream
• 5 tablespoons olive oil, divided
• 12 ounces cremini mushrooms, roughly chopped
• Kosher salt, to taste
• 3 tablespoons bread crumbs
• ½ medium yellow onion, roughly chopped
• 1 tablespoon chopped garlic
• 1 cup cooked farro
• 1 large egg
• ¼ cup finely grated Parmesan cheese
• 3 tablespoons roughly chopped flat-leaf parsley
• 1 tablespoon roughly chopped fresh oregano
• Freshly ground black pepper, to taste
• 2 ounces Gruyère, thinly sliced
• 4 brioche buns, split in half and toasted
• For the Fried Onions
• Canola oil, for frying
• 1 medium yellow onion, cut into ¼-inch rings
• ½ cup all-purpose flour
• Kosher salt, to taste

Directions

1. Make the pickled maitake aioli: Place the mushrooms in a medium bowl. In a small saucepan, combine the white vinegar and sugar, and place over high heat. Bring to a boil then pour over the mushrooms. Add a pinch of salt and let cool to room temperature.
2. In a medium bowl, combine the egg yolk and lemon juice. Whisking vigorously, gradually add the canola oil, drop by drop, until it begins to emulsify. Stir in the cream and garlic. Taste and adjust the seasoning with salt. Strain the mushrooms, discarding the liquid. Finely chop them and fold them into the mayonnaise. Makes about 2 cups. Make ahead: Mayonnaise can be made up to 2 days in advance.
3. Make the mushroom burgers: In a small bowl, combine the oats and cream, and let sit until the oats are fully soaked, 45 minutes.
4. Meanwhile, cook the mushrooms. In a large skillet, heat 2 tablespoons of the olive oil over medium-high heat. Add the mushrooms and cook, stirring occasionally, until they are brown and tender, 8 to 10 minutes. Season with salt and transfer to a medium bowl. Stir in the bread crumbs and set aside.
5. Wipe the skillet clean and place it over medium-high heat. Heat 1 tablespoon of the olive oil and add the onion. Season with salt and cook until soft and golden, 4 to 5 minutes. Add the garlic and continue to cook, stirring often, until fragrant and golden brown, another 4 to 5 minutes. Remove from the heat and transfer the onions to the mushroom mixture along with the soaked oats.
6. In a food processor, add the mushroom-oat mixture and pulse to roughly chop. Add the cooked farro and pulse until coarsely chopped. Add the egg, Parmesan and herbs, and pulse until fairly smooth. (The mixture should have some texture and be slightly sticky.) Transfer to a large bowl and taste, adjusting the seasoning with salt and freshly ground black pepper. Form the mixture into four 4-inch patties. Place them on a plate and chill in the refrigerator, uncovered, for at least 1 hour. Make ahead: Patties can be formed and chilled up to 1 day ahead.
7. While the mushroom burgers are chilling, fry the onions: In a large wide saucepan, add enough oil to reach a depth of ½ inch; place the pan over medium-high heat. In a medium bowl, toss the onions with the flour to coat. Working in 2 batches, fry the onions until dark brown and crispy, 10 to 12 minutes. Using a slotted spoon, transfer the rings to a paper towel-lined plate and season with salt while still hot.
8. In a large skillet, heat the remaining 2 tablespoons of olive oil over medium-high heat. Add the patties and cook until deep brown and crisp around the edges, about 4 minutes per side. Top each patty with cheese, cover and cook until the cheese has melted, about 1 minute.
9. To serve, divide ½ cup of the aioli, the patties and fried onions among the bun slices.