Soy Glazed Butter Mushroom Sandwich - Taste Europe | Butter of France

Ingredients

• 5 tablespoons unsalted European Butter, divided
• 1 medium onion, thinly sliced, about 1⁄8 inch thick
• 1 teaspoon Kosher salt, plus more to taste
• 1 tablespoon mayonnaise
• 1 teaspoon Dijon mustard
• 1 teaspoon brown sugar
• 2 tablespoons good quality light soy sauce, divided
• 5 ounces (about 2 handfuls) baby arugula
• Freshly ground black pepper, to taste
• 1 tablespoon roughly chopped fresh dill
• 1 lemon (or Meyer lemon)
• 2 ounces shaved Parmigiano Reggiano cheese
• 6 slices of sourdough bread
• 6 ounces shiitake mushrooms, de-stemmed and sliced into ½-inch thick pieces