Preheat oven to 400 degrees then line a baking sheet with foil and spray with nonstick spray. Add cauliflower florets then toss with 1 Tablespoon oil, salt, and pepper. Roast for 20-30 minutes or until lightly caramelized then set aside.

Meanwhile, heat remaining 2 Tablespoons oil in a large soup pot over medium heat then add onions and leeks, season with salt and pepper, and then saute until lightly caramelized, 10 minutes. Add garlic then saute until fragrant, 1 minute.

Add chicken broth and dried dill then turn heat up to bring to a boil. Add roasted cauliflower then reduce heat and simmer for 10 minutes. Transfer cauliflower plus a little broth to a blender then blend until very smooth, and then pour back into the soup pot. Bring soup back up to a bubble then add potatoes, cover and reduce heat, and then simmer until potatoes are tender, 8-10 minutes. Stir in fresh dill and a little lemon juice (start with a little, you can always add more.) Taste then add salt and pepper, and then serve.