Place a large frying pan on medium heat add avocado oil and butter.
Once the frying pan is hot add in the onions, mushrooms, balsamic vinegar, salt, and pepper.
Continue cooking onions while stirring every 4-5 minutes for approximately 25 minutes for golden but for deeply caramelized onions you will need approximately 35-40 minutes.
Once onions and mushrooms are cooked add the remaining ingredients.
Cook until the liquid is slightly thickened. Toast the bread until browned.
Slice cheese and add 2 oz of cheese.
Turn oven onto broil.
Place the bread with the cheese on it on a baking sheet in the oven.
Keep a close watch on the bread once the cheese is super melty remove the bread from the oven.
Place a large scoop of soup into bowls or soup cups. Place bread with cheese slice on top and serve.
Decorate with a few leaves of basil.