Cheddar, Roasted Onion and Apple Tartines | MS: Cook What You Have | Umami

Ingredients

• 2 large apples, preferably Granny Smith, peeled, cored and cut into ⅛-inch wedges
• 1 large red OR yellow onion, halved and thinly sliced
• 2 tablespoons extra-virgin olive oil
• Kosher salt and ground black pepper
• 4 tablespoons salted butter, room temperature
• 2 tablespoons Dijon mustard
• 10- to 12-ounce baguette
• 2 teaspoons white wine vinegar OR red wine vinegar
• 6 ounces cheddar cheese, shredded (1½ cups)
• 1 ounce Parmesan cheese, finely grated (½ cup)