For the guacamole, scoop out the flesh of the avocado and add it to a bowl. Sprinkle with salt and pepper and add the lime juice. Mash together with a fork or potato masher until just slightly chunky. Set aside.

Add about 1 tablespoon olive oil to a pan over medium high heat. Once hot, add the onion and cook for several minutes, or until the onion begins to brown. Once slightly browned, add a splash of white wine to the pan and let it cook off. Once the wine has cooked away, place the onions in a small bowl and set aside.

To the same pan over medium heat, add a large handful of spinach. Cook until wilted. Remove from pan and set aside.

Wipe the mushroom clean with a paper towel. Remove the stem and brown scaly insides. Slice the mushroom into long strips. Add another tablespoon of olive oil to the same pan over medium high heat. Add the mushroom strips and stir around to coat the mushrooms in oil. Let cook for several minutes, until the mushroom begins to get soft. Add a splash of balsamic vinegar to the pan, stir, and let cook several more minutes, until the vinegar has cooked away. Remove the mushrooms from the pan and set aside.

To make the sandwich, butter one side of each piece of bread. Place one piece of bread in the same pan over medium heat. Top with the guacamole, tomato slices, one slice of cheese, onions, spinach, mushrooms and then another slice of cheese. Let cook until the bread is crispy and browned. Add the other piece of bread on top, buttered side up. Using a large spatula, quickly but carefully flip the sandwich. Let cook until the other slice of bread is crispy. Smash down a bit to combine everything together, slice and eat.