Mushroom Melt Panini with Gouda and Cheddar

• 1 pound (454 g) cremini mushrooms, sliced 1/4 inch thick
• 200 g shiitake mushrooms, stems removed, sliced 1/4 inch thick
• 1 large onion, sliced into 1/4 inch strips
• 3 tablespoons butter, divided
• 1 to 1 1/2 teaspoons dried thyme or 1 tablespoon fresh thyme
• 3/4 teaspoon salt, divided
• 1/2 teaspoon black pepper
• 1 to 2 tablespoons finely chopped parsley (optional)
• 6 to 8 ounces Gouda, shredded (about 1 1/2 to 2 cups lightly packed)
• 4 ounces aged cheddar, shredded (about 1 cup lightly packed)
• 8 slices sourdough, each 1/2 inch thick (from a round loaf)
• 2 tablespoons Dijon mustard
• 3 tablespoons mayonnaise
• Softened butter for the outside of the bread (about 2 teaspoons total)
• Optional Add-In
• 1 cup shredded, cooked beef (120 to 150 g), warmed but not fried

Instructions

1. 1

   Slice the sourdough into 8 even slices, each 1/2 inch (1.3 cm) thick. Lightly butter the outside surface of each slice. Do not butter the inside.

2. 2

   Heat 1 tablespoon (15 ml) butter in a large skillet over medium heat. Add the onions and 1/4 teaspoon (1.25 ml) salt. Cook for 12 to 15 minutes, stirring occasionally, until soft and lightly golden. Transfer to a bowl.

3. 3

   Add 1 tablespoon (15 ml) butter to the skillet and increase the heat to medium-high. Add half of the mushrooms in a single layer. Do not stir for 3 to 4 minutes to allow browning. Stir once and cook for another 2 to 3 minutes until deeply browned. Season with salt and pepper. Transfer to a bowl. Repeat with the remaining mushrooms using the final tablespoon (15 ml) of butter. When both batches are finished, return all the mushrooms and the onions to the skillet.

4. 4

   Add the thyme, the remaining salt, and black pepper. Cook for 2 minutes to combine the flavours. If using parsley, stir it in now. If adding the optional beef, add the shredded beef and warm it for 1 to 2 minutes without browning it. The filling should be dry, not watery. If any liquid remains, cook for 1 to 2 minutes longer to evaporate the excess moisture.

5. 5

   Mix the Dijon and mayonnaise in a small bowl. Spread a thin layer on the inside of each bread slice. Divide the shredded Gouda and cheddar into eight portions. For each sandwich, place one portion of cheese on one bread slice, add 3/4 to 1 cup (180–240 ml) of mushroom filling, then top with another portion of cheese. Finish with the second slice of bread, buttered side facing outward. Clarification: The cheese goes on both sides of the mushroom filling inside the sandwich. The bottom cheese layer helps anchor the filling and melt evenly; the top layer helps seal the sandwich. This prevents the mushrooms from slipping and improves melt and structure.