4 Portobello mushroom caps, stems removed, wiped clean
Olive oil cooking spray
1 teaspoon kosher salt, divided
¼ teaspoon black pepper, divided
10oz frozen chopped spinach
2 tablespoons olive oil, divided
1 small onion, finely chopped (6oz)
1 tablespoon minced garlic
¼ cup grated Parmesan
Preheat broiler, setting temperature to high. Set oven rack in the middle of the oven. Line a baking sheet with foil.
Spray the mushroom caps with olive oil cooking spray on both sides. Sprinkle with ½ teaspoon kosher salt and ⅛ teaspoon black pepper. Broil 5 minutes on each side, or until just tender.
Meanwhile, defrost spinach in the microwave according to the directions on the package (my package said to cut a small slit in the wrapper, place on a microwave-safe plate and microwave for 4-6 minutes). Place in a colander to drain. When cool enough to handle, press on the cooked spinach with your hands and extract as much water as possible out of it. Repeat this until you're sure you can no longer extract more water.
Remove mushrooms from oven. Heat 1 Tbsp. olive oil in a large skillet over medium heat, about 3 minutes. Add the onion and cook 5-7 minutes, stirring occasionally, until golden. Add the garlic, spinach, ½ teaspoon kosher salt and ⅛ teaspoon black pepper and cook, stirring to combine everything, 1-2 more minutes. Remove from heat and allow to cool a few 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 it's piled high on top of the mushrooms.
Place back under the broiler for 2-3 minutes, or until filling is golden.