Finely mince the cremini mushrooms until they reach a texture similar to the ground beef.
In a medium bowl, thoroughly mix the ground beef, minced mushrooms, sea salt, black pepper, and garlic powder.
Shape the mixture into two thick patties, ensuring they are tightly packed so they don't fall apart on the grill.
Brush a grill pan with olive oil and heat over medium-high heat until shimmering.
Place the patties on the grill and cook for 5-6 minutes per side, or until the internal temperature reaches 160°F.
Prepare the lettuce wraps by layering two butter lettuce leaves for each burger to create a sturdy base.
Place the grilled patties onto the lettuce leaves and top with avocado slices, tomato, and red onion.
Finish with a drizzle of yellow mustard and fold the lettuce around the burger to serve.