Heat oil in a large pot over medium-high heat, sauté onions for 8-10 minutes until tender. Add the garlic and cook until it's just fragrant. Add kohlrabi and cook for 2 minutes. Add stock, water, cashews, bay leaves, thyme, white pepper and salt. Bring to a boil, reduce heat to low to simmer, and cook for 20-25 minutes until kohlrabi is tender. Remove bay leaves.
Using an immersion blender, puree soup until smooth. If you're using a blender: let the soup cool first before blending and blend in small batches. Add steamed potatoes and sautéed mushrooms. Season to taste with salt, pepper and dried chili flakes. Serve in bowls with blanched kohlrabi greens and olive oil.