1 cup pearl barley
2 1/2 cups low-sodium mushroom (vegetable or chicken) stock
6 portobello mushrooms
4 tablespoons extra virgin olive oil, divided
2 medium carrots, halved lengthwise, chopped
3 medium celery stalks, chopped
1 medium onion, chopped
1 clove garlic, minced
Kosher salt, to taste
Freshly ground black pepper, to taste
2 tablespoons minced fresh flat-leaf Italian parsley
1 tablespoon fresh thyme leaves
1/4 cup grated Parmigiano-Reggiano, plus extra for garnish
6 slices provolone cheese, or to taste
In a medium pot, add barley and stock. Bring to a boil over medium heat, cover, and reduce heat to low. Simmer for 30-40 minutes, until al dente. Remove from heat and drain any remaining liquid. Set aside to cool.
Preheat oven to 400°F. Line a baking sheet with parchment paper.
Remove portobello stems, finely chop, and set aside. Removegills from caps and discard. Use 1 tablespoon of olive oil to coat both sides of caps. Arrange hollow-side up on prepared baking sheet. Bake for 15-20 minutes, turning once halfway through, until softened. Remove from oven and allow to cool.Leave oven on.
In a large nonstick pan, heat remaining olive oil over medium heat. Add carrots and celery and cook for 10 minutes, stirring often, until carrots are softened. Add onion, garlic, and chopped mushroom stems. Season with salt and pepper. Continue cooking for 3 minutes, until onions are softened. Remove pan from heat and set aside to cool.
Place oven rack on uppermost position. Set oven to broil.
To make stuffing, add vegetables, parsley, thyme, and Parmigiano-Reggiano to barley. Stir to combine.
Make sure mushrooms are hollow-side up. Scoop about 1 cup of stuffing into each cap. Top with provolone slices and extra Parmigiano-Reggiano. Transfer to oven and broil for 5 minutes, or until cheese melts.