Preheat your oven to 375°F (190°C). This step is crucial as it ensures the mushrooms roast evenly. Once your oven is ready, it’s time to prepare the mushrooms.
Prepare the mushrooms. Place the portobello mushroom caps on a parchment-lined baking sheet. Drizzle with your choice of oil and sprinkle the kosher salt on both sides. This will enhance the taste of the mushrooms while they roast.
Roast the mushrooms. Flip the mushrooms so the gill side is facing down and roast them for about 30 to 35 minutes. You want them to be slightly golden and tender. Once done, remove them from the oven and allow them to cool slightly.
Slice the caps horizontally to create even halves. Be careful to cut directly through so both sides are even. This will make them easier to stuff.
Make the cheese filling. In a medium bowl, combine the mozzarella cheese, cheddar cheese, onion powder, and garlic powder with one of the eggs. Mix thoroughly until combined. Form this mixture into four disks, slightly smaller than the mushroom caps.
Stuff the mushrooms. Place a disk of the cheese filling between the halves of each mushroom cap. Press down gently to secure the filling inside. Once done, refrigerate the stuffed mushrooms until you are ready to cook.
Create a breading station. In three separate shallow bowls, set up your breading station: one with the flour, one with whisked eggs (using the remaining three eggs), and the last with panko breadcrumbs.
Dredge each stuffed mushroom in the flour first, shaking off any excess. Next, dip it in the whisked eggs, ensuring it’s fully coated, and finally, press it into the panko breadcrumbs to create a crispy layer.
Heat the oil. In a large sauté or cast-iron pan, pour in about half an inch of oil and heat over medium-high heat. If you have a candy thermometer, you want it to read about 350°F (175°C).
Fry the burgers. Carefully place the breaded mushrooms in the hot oil. Fry them for about 3 to 4 minutes per side, or until golden and crisp. Ensure they’re cooked evenly and golden brown.
Drain excess oil. Once cooked, transfer the burgers to a plate lined with paper towels to absorb any excess oil.
Make the faux Shack Sauce. In a small bowl, combine the mayonnaise, dijon mustard, ketchup, pickle juice, and a pinch of cayenne pepper if you’re feeling adventurous! Stir until fully combined.
Assemble the burgers. Place each shroom burger on the bottom half of a bun, then layer on avocado slices, tomato, and lettuce. Drizzle with the Shack Sauce if desired, then top with the other half of the bun.
