Spinach and Parmesan Stuffed Portobellos

Heat the oven (with the oven rack in the middle) to 425°F. Break off and discard the stems at the base of the mushrooms. Using a damp paper towel, wipe off any visible dirt.

Measure 2 tablespoons of the oil into a small bowl. Using a pastry brush, brush both sides of the mushroom caps with the oil. Place them, stem-side up, in a rimmed sheet pan or baking dish. Roast until the mushrooms are tender and the centers can be easily pierced with the tip of a paring knife, 20 to 25 minutes.

Meanwhile, slice the onions into half moons. Place a large skillet on the stove and turn the heat on to medium-high. Add the remaining 2 tablespoons oil and heat until it shimmers (about 1 minute). Add the sliced onion and cook, stirring often, until tender and golden brown, 8 to 10 minutes (you may have to turn down the heat as it cooks to avoid scorching). While the onions cook, wash and spin dry the spinach.

Add the spinach to the cooked onion and sprinkle with salt and pepper. Place a lid over the spinach until it starts to wilt and shrink, about 1 minute. Remove the lid and stir until the spinach is completely wilted and any water is evaporated, 1 to 2 minutes more. Remove from heat.

Dividing evenly, fill the roasted mushroom caps with the spinach mixture. Sprinkle the grated Parmesan over the tops.
Adjust the oven rack to about 4 inches from the top. Turn your broiler on to high. Broil the mushrooms until the Parmesan melts and bubbles, about 2 minutes.