In a large soup pot add a lug of olive oil over medium flame. Add the rinsed leeks and saute until wilted, bout 20 minutes.
Add 2 sage sprigs and the butternut squash cubes to the leeks and cover with the water. Bring to a simmer, add the sea salt and cover with a lid. Cook for about 30 minutes until the butternut squash is cooked through but still al dente not mushy.
Using a hand held immersion blender puree the soup to your linking, but try to keep some rustic textures going. Adjust seasonings to your taste with more sea salt at the point.
Meanwhile melt the butter in a large skillet on medium low flame. Add the sage leaves and crisp them up in the butter for about a minute making sure not to burn them
Transfer the sage to a paper towel lined plate to drain. Sprinkle with a pinch of sea salt.
Laddle the soup into bowls and serve with the crispy sage leaves on top and a good sprinkling of red pepper flakes and freshly cracked black pepper.
Serve garnished with a drizzle go heavy cream and crusty bread.