Combine the cauliflower, stock, cream, and salt (add half the amount and then adjust up in the final puree if you want more) in a large pot.

Bring to a boil, uncovered and cook until there is almost no liquid left and the cauliflower is just beginning to caramelize (this is key for a good texture – too much liquid results in a runny puree), about 30 minutes on medium-high heat.

While the cauliflower cooks, prepare the remaining vegetables. Melt half the butter in a large nonstick pan over medium heat. Add the leeks and cook until soft and the whites are translucent, about 10 minutes, stirring occasionally. Add the garlic and cook until fragrant, about 1 minute more. Add the cabbage and cook, until soft and wilted and the vegetables are beginning to brown, about 5-10 more minutes. Set aside.

Using an immersion blender, puree the now soft cauliflower until completely smooth. You can also transfer everything to a traditional blender and puree. Stir in the cooked and reserved vegetables. Season to taste with salt and pepper. Transfer to a serving bowl and top with remaining butter. Scatter green onion over top.