1 1/2 cups boiling water
1 ounce dried porcini mushrooms
4 tablespoons (1/2 stick) unsalted butter
1 medium onion, diced
2 medium stalks celery, diced
2 medium carrots, peeled and diced
1 teaspoon kosher salt
1/4 teaspoon freshly ground black pepper
3/4 cup pearled barley
2 tablespoons all-purpose flour
12 ounces cremini mushrooms, stems removed and cut into 1/4-inch-thick slices
6 cups low-sodium vegetable broth
Place the water and porcinis in a medium heatproof bowl; set aside to soak.
Melt the butter in a Dutch oven or large pot over medium heat. Add the onion, celery, carrots, salt, and pepper. Cook, stirring occasionally, until the vegetables are soft and tender, about 8 minutes. Add the barley and cook, stirring occasionally, until lightly toasted, about 3 minutes.
Add the flour, stir so it coats the barley and vegetables, and cook for 2 minutes. Add the porcini mushrooms and their soaking liquid (avoid any grit at the bottom of the bowl), cremini mushrooms, and vegetable broth. Bring to a boil, then reduce the heat and simmer until the barley is tender, 20 to 25 minutes.