In a bowl, combine the shiitake and cremini mushrooms with soy sauce, balsamic vinegar, olive oil, salt, and black pepper. Let marinate for at least 30 minutes.

Caramelize the Onions

Slice the onions thinly and heat a pan over medium heat with a drizzle of olive oil. Add the onions and cook until golden and caramelized, about 15-20 minutes.

Make the Basil Mayo

In a blender, combine the fresh basil and mayonnaise. Blend until smooth. Adjust seasoning with salt if needed.

Cook the Mushrooms

In the same pan used for the onions, add the marinated mushrooms and cook over medium-high heat until they are tender and browned, about 8-10 minutes.

Assemble the Melt

Preheat the oven to 180°C (350°F). Slice the house rolls in half and spread a generous amount of basil mayo on each side. Place the caramelized onions, cooked mushrooms, and then top with Gruyere cheese.

Bake

Place the assembled sandwiches on a baking sheet and bake in the preheated oven for 10-15 minutes, or until the cheese is melted and bubbly.

Serve

Remove from the oven, let cool for a minute, then serve warm.