Vegetarian Borscht

1-ounce (30 g) dried porcini mushrooms

2 tablespoons vegetable oil

1/2 pound white mushrooms, trimmed, wiped clean, and sliced 1/4 inch thick

1 large onion, cut into 1/4-inch dice

10 small or 7 to 8 medium beets, with greens, peeled, quartered, and cut across into 1/4-inch slices

2 medium carrots, peeled and cut across into 1/4-inch rounds

1 medium parsnip, peeled and cut across into 1/4-inch rounds

1 very small or 1/2 large celery root (about 3/4 pound), peeled and cut into 1/2 inch cubes

1 1/2 pounds mashing potatoes, peeled and cut into 1/2-inch cubes

1/2 small white cabbage (about 3/4 pound), cored and shredded

3 large cloves garlic, smashed, peeled, and very finely chopped

3 tablespoons tomato paste

1 medium bunch dill, fronds only, coarsely chopped

1/4 cup sugar

1/2 cup cider vinegar

2 tablespoons kosher salt

Freshly ground black pepper, to taste

Coarsely chopped dill

Sour cream

Boiled potatoes, optional