Preheat oven to 350 degrees F (175 degrees C).

Place the whole cauliflower head in a 9x13 inch Pyrex baking dish.

In a small bowl, combine 2 tablespoons of olive oil and salt.

Rub the olive oil and salt mixture all over the cauliflower head, ensuring it is evenly coated.

Pour 1/2 cup of water into the baking dish around the cauliflower.

Bake uncovered for 1 1/2 hours, or until a knife easily pierces the core of the cauliflower.

Remove the roasted cauliflower from the oven and let it cool slightly.

Once cooled, coarsely chop the roasted cauliflower and set it aside.

Heat the remaining 2 tablespoons of olive oil in a large saucepan or pot over medium heat.

Add the diced onion to the saucepan and cook until it becomes soft and translucent, approximately 15 minutes.

Add the chopped roasted cauliflower and 4 cups of water or stock to the saucepan.

Bring the mixture to a simmer and cook until the cauliflower is very soft, about 10 minutes.

Carefully transfer the soup in small batches to a Vitamix or high-speed blender.

Puree each batch until completely smooth and creamy.

Pour the pureed soup back into the pot.

Gently heat the soup over low heat until it reaches a simmer.

Serve hot.