Preheat the oven to 400° F. Cut the cauliflower into small florets. If desired, thinly slice the stem as well and add to the florets. Transfer the cauliflower to a baking sheet. Drizzle with 2 tablespoons of olive oil and season with 1 teaspoon kosher salt and 1/4 teaspoon pepper. Toss to coat. Roast the cauliflower for 25 to 30 minutes until tender and beginning to brown.
In a Dutch oven or large pot, heat the remaining 2 tablespoons of olive oil over medium high heat. Add the diced onion and sauté for about 2 minutes until the onion begins to soften. Add the sliced leeks and continue to sauté until both the onion and leeks are softened and beginning to brown. Add the minced garlic and sauté for another minute.
Add the vegetable broth to the pot and bring to a simmer. Season with the remaining teaspoon of kosher salt and 1/4 teaspoon pepper. Add the roasted cauliflower to the broth and bring to simmer. Cook the soup at a low simmer for 10 to 15 minutes until all the vegetables are very tender.
Transfer the soup to a blender in batches and blend until smooth. Alternatively, use a hand blender to purée the soup. Return the puréed soup to the pot and place over medium low heat. Taste and adjust the seasoning if needed. Stir in the heavy cream and parmesan cheese.
Gremolata
In a small bowl, combine the finely minced parsley, fine minced garlic, and lemon zest. Add the olive oil and lemon juice along with the kosher salt, pepper, and red pepper flakes. Whisk together and set aside.
Toasted Breadcrumbs
Tear the bread into bite sized pieces and add to a food processor. Pulse until the bread has broken down into crumbs. Melt the butter in a skillet over medium heat. Add the bread crumbs and season with the kosher salt and pepper. Cook slowly for 5 to 7 minutes, stirring regularly until the crumbs begin to brown. Transfer to a plate and cool.
To Serve
Top each serving with swirls of the gremolata and a sprinkle of toasted bread crumbs. Enjoy!