- 4 large portobello mushroom caps (about 1½ pounds total)
- ¼ cup olive oil, plus more for oiling the baking dish
- ½ cup finely minced shallots (from 3 medium-size shallots)
- 1½ cups finely minced fennel (2 small bulbs)
- 4 cloves garlic, finely minced
- 6 ounces shiitake mushrooms, stems discarded, caps minced (about 1 cup)
- 12 ounces baby spinach, cut into ribbons (about 8 cups)
- ¼ cup minced fresh flat-leaf parsley
- 3 tablespoons minced fresh tarragon
- 1¼ cups freshly grated Parmesan cheese
- Salt and freshly ground black pepper
- Wipe the portobello mushroom caps clean with a damp paper towel. Using a small paring knife or teaspoon, remove the gills to create a shallow depression for the stuffing. Place the portobellos on a lightly oiled baking dish and set aside.
- Position a rack in the center of the oven and preheat the oven to 400°F.
- Heat the olive oil in a large skillet over medium-low heat. Add the shallots and fennel and cook until soft, 8 to 10 minutes.
- Add the garlic and shiitake mushrooms and cook until the moisture has evaporated, about 10 minutes. Stir in the spinach and cook until it wilts, about 5 minutes, working in batches, if necessary.
- Add the parsley, tarragon, and ½ cup of the Parmesan cheese and stir to combine. Cook until the cheese melts, about 1 minute. Season the filling with salt and pepper to taste.
- Spoon the filling into the portobellos, mounding it slightly. Sprinkle 3 tablespoons of the remaining Parmesan cheese over each mushroom. (The mushrooms can be prepared up to this stage 4 hours ahead. Refrigerate them, covered until ready to bake.)
- Bake the mushrooms until they are tender but not soggy and the cheese on top has browned, 20 to 25 minutes. Serve immediately.