Step 1In a 5-quart heavy-bottom pot heated over high heat, melt 1 tablespoon butter. Stir in the onions and saute until they begin to soften, about 6 minutes. Reduce the heat to low and add the garlic. Continue cooking, stirring occasionally, until the onions are golden and caramelized, about 20 minutes. Transfer the onion mixture to a bowl and set aside.
Step 2Melt the remaining tablespoon butter with the olive oil in the same pot over medium heat. Stir in the mushrooms. Cook the mushrooms just until they begin to soften, about 6 minutes. Stir in the balsamic vinegar, then cover the pot and lower the heat. Continue cooking until the mushrooms are tender, dark and jammy, about 10 minutes, stirring occasionally to make sure they don't stick to the pot.
Step 3Stir the onion mixture back in with the mushrooms, then add the vegetable broth.
Step 4Tie the star anise, cloves, peppercorns, coriander and sage leaves in a small square of cheesecloth to form a sachet. Stir the sachet in with the broth along with the bay leaf, parsnips and turnip. Simmer, covered, for 30 minutes, then stir the beets and sauerkraut into the pot. Cover and continue to simmer until all of the vegetables are tender, 30 to 40 minutes. Season to taste with salt, pepper and lemon juice.
Step 5Ladle the borscht into bowls and serve topped with a teaspoon of sour cream and a sprinkling of cumin.