Preheat your oven to 400°F (200°C). Spread the chopped carrots, parsnips, and kohlrabi evenly on a baking sheet. Drizzle with olive oil and toss to coat.
Roast the vegetables in the oven for about 25-30 minutes, until they are golden brown and caramelized at the edges, filling your kitchen with a warm, sweet aroma.
Meanwhile, heat a large pot over medium heat. Add a splash of oil and sauté the minced garlic with chopped sage until fragrant, about 1 minute, until the garlic turns golden.
Add the roasted vegetables to the pot and pour in the vegetable broth. Bring the mixture to a gentle simmer and cook for 10 minutes to let the flavors meld.
Use an immersion blender to carefully blend the soup until smooth and creamy. Alternatively, transfer the hot mixture to a blender and purée in batches, then return to the pot.
Taste the soup and season with salt and pepper as needed. If the soup is too thick, add a splash more broth until you reach your preferred consistency.
Warm the soup just until hot, then ladle into bowls. Garnish with a few fresh sage leaves or a drizzle of olive oil if desired. Serve immediately for a comforting, vibrant fall soup.