Preheat oven to 425F.
Toss cauliflower, garlic, onion, with 3 tbsp of olive oil, rosemary, salt and pepper on a large baking sheet, and roast for 45 min - 1 hour, turning half way through. Keep an eye out to prevent from burning.
In a large pot, heat 2 tbsp of butter over medium heat.
Add in sliced leek, and saute for 4-5 minutes or until softened.
Add in the potatoes and saute for another 2-3 minutes.
Remove the rosemary from the roasted vegetables, and add all the vegetables into the pot. Toss well then pour in vegetables stock.
Bring everything to a simmer, add in 2 tbsp of miso paste, and cook for another 10 minutes or until potatoes are soft.
Use an immersion blender or a regular blender and blend everything until it is smooth.
Optional: stir in heavy cream for extra richness.
Taste and adjust seasoning with more salt and pepper.
