Vadouvan spice, a French-style curry blend, lends a subtle heat to this soup.
Heat oil in large pot over medium-high heat. Add carrots, leeks, and baking soda. Cover, and cook 15 minutes, stirring occasionally, or until vegetables are tender and browned in spots. Stir in vadouvan spice, and cook 30 seconds. Add 4 cups water, and bring to a simmer. Remove from heat, purée with immersion blender, and season with salt, if desired. Serve sprinkled with pistachios.