Preheat oven to 375 degrees. On a cookie sheet place all of the mushrooms, cap side down and drizzle with olive oil and season with salt and pepper. Flip the mushrooms over so the cap is up and repeat with the oil and salt and pepper. Bake for 10 minutes or until the mushrooms are almost tender.
While the Mushrooms are in the oven, heat 2 teaspoons of olive oil in a saute pan and cook the onions until they are just starting to caramelize. Add in the spinach and cook until the spinach has warmed through. Add in the goat cheese and 1 teaspoon of salt and stir to combine.
Remove the mushrooms from the oven and turn the oven to broil. Turn the mushrooms over so the gill side is up and place 2 tablespoons of filling onto the mushroom. Spread slightly so there is a even layer. Top each mushroom with 2 tablespoons of panko bread crumbs and drizzle the bread crumbs with olive oil. Broil for 1-2 minutes or until the bread crumbs are golden.
Serve warm and enjoy!