This recipe achieves that wonderful double bonus of being quick to make and quick to reheat. Portobellos are filling while also fulfilling any dietary restrictions; they cook quickly and add a meaty flavor without being obtrusive; and, lesser known, they hold up in the freezer like a dream. You can heat these up as a quick vegetarian side or have them as an entire meal. Either way, you'll have something delicious on the table in minutes.
Swap option: This recipe can be swapped endlessly with other cheeses like blue cheese, cream cheese, mozzarella or feta. If you don't have shallots, onions or scallions work fine. And any hearty lettuce like kale, radicchio or romaine could replace the spinach.
Place the top rack in the oven 8 inches or so from the top. Turn the broiler on. Place the mushrooms on an aluminum foil-lined sheet pan. Coat the mushrooms on both sides with olive oil and salt, and place the smooth side up on the pan and into the oven. Cook for 7 minutes.
Combine the shallots with the goat cheese and lemon juice, whisking well. Fold in the spinach and an additional dash of salt.
If you are eating now: Remove the mushrooms from the oven, flip and top with the goat cheese mixture. Cook another 4 minutes and serve hot.
If you are freezing for later: Remove the mushrooms from the oven and allow them to cool completely. Flip them over and top with the goat cheese mixture. Wrap each in aluminum foil or plastic wrap and place them all in a freezer-safe container.
When you are ready to eat, remove the mushrooms from their wrapping and place on a plate in the microwave. Microwave for 1 minute at a time, until hot and ready to eat.