In a medium bowl, combine the shredded chicken, mashed avocado, Greek yogurt, lime juice, garlic powder, onion powder, salt, and pepper. Mix until well combined.
Heat a skillet over medium heat and add the olive oil.
Spread the chicken and avocado mixture evenly on two slices of bread. Top with shredded cheddar cheese.
Place the remaining slices of bread on top to form sandwiches.
Carefully place the sandwiches in the skillet and cook for about 3-4 minutes on each side, or until the bread is golden brown and the cheese is melted.
Remove from the skillet, slice in half, and garnish with fresh cilantro if desired.