Preheat your oven to 350 degrees Fahrenheit.
Slice a baguette into thin slices and place onto a shallow baking sheet.
Brush 1-2 tbs of olive oil over the tops of the bread. Bake in the oven for 6-8 minutes or until the slices begin to brown.
Remove the bread from the oven and then turn the oven off.
Add small pieces of swiss cheese to each crostini and place back in the off oven for another 3-4 minutes or until the cheese starts to melt. Remove and set aside.
In a medium to large saute pan, add 2 tablespoons of olive oil and finely diced onion.
Saute for 6-8 minutes or until the onions have started to brown.
Add the sliced mushrooms and dried thyme to the pan. Cook for 10-12 minutes or until the mushrooms have released all of their liquid and have started to brown.
Season with a little salt and pepper.
Then add the balsamic vinegar. Make sure to use a quality balsamic vinegar.
Once the balsamic vinegar has been absorbed remove the mushrooms from the heat. Add the parsley and stir in.
Then top each Swiss cheese covered crostini with mushroom mixture.