Mushroom Lentil Burgers

INGREDIENTS

• 1 tablespoon extra-virgin olive oil, divided
• 3/4 cup chopped onion
• 2 garlic cloves, chopped
• 5 regular or whole-wheat hamburger buns, toasted and divided
• 6 ounces presliced cremini mushrooms
• 1 (8-ounce) pouch precooked black beluga lentils (such as Archer Farms)
• 4 teaspoons Dijon mustard, divided
• 2 teaspoons chopped fresh thyme
• 1/2 teaspoon freshly ground black pepper
• 1/4 teaspoon kosher salt
• 1 large egg, lightly beaten
• 1 ounce goat cheese, crumbled
• 1 1/2 tablespoons canola mayonnaise
• 1/2 cup baby arugula