Finnish Reindeer Burger from Ramsay Around The World

Ingredients

• 5 ounces ground reindeer (venison works as well)
• 3 ounces ground smoked reindeer (venison works as well)
• 1 egg
• Olive oil
• Kosher salt
• Freshly cracked black pepper
• 1 cup chanterelle mushrooms, cleaned
• 1 small yellow onion, sliced into ½-inch rounds
• Unsalted butter
• 2 slices gouda cheese
• 1 brioche bun, sliced and toasted on both sides
• Mayonnaise
• Dijon mustard
• Butter lettuce leaves
• Flaky salt
• 1 large tomato, sliced