Heat oil in a soup pot over medium flame.

Add onion and leeks and sauté for 15–20 minutes until soft and opaque. Add garlic and sauté for about three minutes until fragrant.

Add cauliflower, squash, salt, and pepper. Add water until liquid reaches just until one inch (two centimeters) below the top of the vegetables. Bring to a boil, then reduce heat, cover, and simmer for one hour, stirring occasionally.

Blend soup with an immersion blender until puréed. If the soup is too thick, add a little boiling water. If it’s too thin, simmer uncovered for a few minutes to thicken.

Preheat oven to 450 degrees Fahrenheit (230 degrees Celsius).

Grease a baking sheet and throw on all ingredients.

Toss to cover and bake for 15 minutes, or until starting to crisp.

Garnish the soup with mushroom croutons just before serving.