Porcini Mushroom Soup

3/4 ounces dried porcini mushrooms (1 cup)
6 cups tepid water plus 2 cups hot water, divided

1 medium onion, finely chopped
1/2 stick unsalted butter

2 celery ribs, finely chopped

1 medium carrot, finely chopped

3 garlic cloves, finely chopped
1 1/2 pounds white mushrooms, sliced or quartered
1 (15-ounce) can diced tomatoes, drained
2 tablespoons finely chopped flat-leaf parsley

2 tablespoons finely chopped dill

Soak Porcini in 2 cups hot water 15 minutes. Cook onion in butter with 1 teaspoon salt in a heavy medium pot over medium heat, stirring occasionally, until onion is golden brown, 10 to 12 minutes. Add celery, carrot, and garlic and cook, stirring occasionally, until softened, about 5 minutes. Transfer porcini with a slotted spoon to pot and strain soaking liquid through a fine-mesh sieve into a large glass measure. Add white mushrooms to pot with 1 teaspoon salt and 1/2 teaspoon pepper. Cook, stirring, until mushrooms are tender, 6 to 8 minutes. Stir in tomatoes, remaining 6 cups water, and porcini-soaking liquid. Simmer, partially covered, 30 minutes. Purée 1 cup vegetables and 1 cup liquid in a blender (use caution when blending hot liquids), then return to pot. Stir in parsley, dill, and salt to taste.