1. Make the cheese sauce.
2. Toast the bread.
3. In a small pan over medium-high heat, heat oil. Add onion and saute until it starts to caramelize. Add your mushrooms and continue to saute until all is golden brown.
4. Spread cheese sauce on one side of each bread slice. On one slice of bread, layer the cheese sauce with avocado slices, then mushrooms. On the other slice of bread, layer the cheese sauce with the tomatoes. Put the two sides together. Your mouth will thank you later!
Source of recipe: come up with it while playing in the kitchen