- 3 tablespoons olive oil
- 2 sweet onions, peeled, halved, and thinly sliced
- Kosher salt and freshly ground black pepper
- ½ bunch parsley
- 7 sprigs thyme, plus fresh thyme leaves, for serving
- 2 bay leaves
- 2 teaspoons black peppercorns
- 2 medium carrots, roughly chopped
- 3 stalks celery, with tops, roughly chopped
- 1 ½ cups dried shiitake mushrooms
- 8 cups water
- 4 fresh shiitake mushrooms, stemmed and sliced paper thin (optional)
Preheat a large pot over medium heat. Add olive oil, swirl to coat, and add onions. Season with salt and pepper and cook, stirring, until jammy and caramelized, adjusting heat as necessary to prevent scorching, about 30 minutes. Add 1 to 2 tablespoons of water from time to time to deglaze the pan and hasten caramelization.
Make a bouquet garnis by wrapping parsley, thyme, bay leaves, and black peppercorns in cheesecloth and tying with kitchen twine. Add to pot, along with carrots, celery, and dried shiitakes. Add water and bring to a boil. Maintain a brisk simmer and let cook, about 45 minutes, until mixture is reduced by one-third. Pull out shiitake mushrooms with a slotted spoon and transfer to a cutting board. Thinly slice.
Strain broth. Do not push on solids. Return sliced mushrooms to pot and season to taste with salt and pepper. Rewarm before serving, if necessary.
Divide among serving bowls and garnish with thyme and fresh shiitakes, if using. Serve, with cheesy popovers.