Preheat a nonstick frying pan over medium-low heat.
Add the avocado to a small mixing bowl. Add the juice from the lime wedge, a pinch of salt, and garlic powder, then mix together.
Mix in the red onion and cilantro.
Spread half of the avocado mixture on one slice of bread. Spray the pan with olive oil spray and add the bread avocado side up. Top the avocado with tomato, cheese and a second piece of bread.
Cook for 3 minutes on each side, or until the bread is golden and crispy.
Repeat the steps to make the second sandwich.