Flat Top Porcini-Dusted Smashed Burger - Traeger Grills

Ingredients

• 1 Tablespoon kosher salt
• 1 1/2 Teaspoon freshly ground black pepper
• 1 1/2 Teaspoon dried porcini powder
• 1/4 Cup mayonnaise, plus more for the buns
• 2 Tablespoons ketchup
• 2 Tablespoons finely diced cornichons
• 1 Tablespoon finely diced white onion
• 1/4 Teaspoon onion powder
• 1/4 Teaspoon granulated garlic
• 4 hamburger buns
• 2 Pounds ground beef, 80% lean
• 1 Tablespoon canola oil or other neutral flavored oil with a high smoke point
• 1/4 Cup thinly sliced white onion (optional)
• 8 Slices American cheese
• 1/2 red onion, thinly sliced
• 1 tomato, sliced