Preheat the oven to 400° and line a large baking tray with parchment paper.
Place the chopped cauliflower into a large bowl and toss with 1 tablespoon of olive oil and a pinch of salt.
Arrange the florets on the lined baking tray and place in the oven for 25-30 minutes, or until golden brown and easily pierced with a fork. Remove from the oven, set aside and, pick out 4-6 pretty florets for garnish, if using.
When the cauliflower is just about done roasting, heat a large dutch oven over medium-high heat with the remaining 1 tablespoon of olive oil.
Once the oil is warmed, add the sliced leeks, celery and, a pinch of salt. Cook for about 5 minutes until they soften and become translucent. Add the garlic and cook for 2 minutes, until fragrant, stirring often.
Add the roasted cauliflower, vegetable broth and, water into the pot and bring the mixture to a boil. Once boiling, reduce heat and let simmer for 20 minutes.
Remove from the heat, and let cool for a few minutes.
Carefully transfer the mixture into a high-speed blender with the lemon juice, pepper and, vegan butter. Blend until completely smooth.
Taste and adjust seasonings as needed.
Pour soup into bowls and garnish with fresh herbs and roasted cauliflower florets, if desired.