Preheat the broiler, setting the temperature to high (500°F). Set an oven rack in the middle of the oven. Line a broiler-safe, rimmed baking sheet with foil.
Gently wipe the mushrooms clean with a damp paper towel. Twist or cut off the stem of each mushroom. Use a small spoon to scrape out the gills and discard them.
Spray the mushroom caps with olive oil on both sides. Sprinkle them with ½ teaspoon kosher salt and ⅛ teaspoon black pepper. Broil until just tender, about 4 minutes per side. Place them upside down on paper towels to drain.
Meanwhile, defrost the spinach in the microwave according to the directions on the package. Place the defrosted spinach in a colander to drain. Repeatedly press on the spinach with the back of a large spoon to remove as much water as possible.
Heat the olive oil in a large skillet over medium heat. Add the onion and cook it for 5-7 minutes, stirring occasionally, until golden. Add the garlic, spinach, ½ teaspoon of kosher salt, and ⅛ teaspoon of black pepper. Cook, stirring, for 1-2 more minutes. Remove the skillet from the heat and allow it to cool for a couple of minutes, then mix in the parmesan.
Evenly distribute the spinach mixture among the mushrooms. Don't press down on the filling - it's prettier when piled high on top of the mushrooms.
Place the stuffed mushrooms back under the broiler (on the middle rack) for 2-3 minutes until the filling is golden. Serve immediately.
Video
Notes
We're setting the oven rack in the middle of the oven because we don't want the mushrooms too close to the heating element.
Scraping out the gills makes room for the filling, improves the mushroom's texture, and helps reduce excess water. The mushrooms will still release quite a bit of water as you broil them, so once they're done, place them upside down on paper towels to drain.
I can't stress strongly enough how important it is to thoroughly drain the defrosted spinach. You want no traces of water left! So, place it in a colander and press on it repeatedly with the back of a large spoon until no water comes out.
You can keep the leftovers in the fridge, in an airtight container, for 3-4 days. Place them on paper towels and replace the towels daily to absorb any extra liquid. Reheat the leftovers in the microwave, covered, at 50% power. I don't recommend freezing the leftovers.