Preheat the oven to 425 degrees Fahrenheit, and adjust the oven rack to the middle position. Line a rime baking sheet with parchment paper.
Add cauliflower florets with half of the oil to the baking sheet. Toss well until the florets have all been lightly coated. Season lightly with half of the salt. Bake at 425 degrees for 25-30 minutes, until the florets are fork tender and browning. Be sure to flip and toss them about halfway through the cook time.
Once the cauliflower has about 5-7 minutes left of cook time, warm a small stock pot or Dutch oven over medium heat with the rest of the oil for 1 minute.
Add onions and the rest of the salt. Sauté for 5-7 minutes until soft and almost translucent. Add garlic and sauté for 30 seconds to 1 minute, until fragrant but stirring frequently.
Add broth, roasted cauliflower florets, and fresh herbs to the pot. Increase the heat to medium high until the mixture reaches a hard simmer. Reduce the heat to low so the soup is now gently simmering.
Cook, stirring here and there, so the flavors all begin to marry together for about 15 minutes. If you'd like the soup to not be as chunky, feel free to add 1 cup of filtered water.
Transfer the soup mixture into the high-speed blender. Blend on medium-high until smooth, about 30 seconds or so.
Add butter, lemon juice, and spices. Do not add any additional salt until the mixture is blended through and you taste it. Blend on medium until everything has been well incorporated. Taste for seasoning and add additional salt. I ended up adding another 1 to 1 1/2 teaspoons.
Pour the soup into serving bowls and garnish with pancetta (omit for vegan) and fresh parsley.