Preheat the oven to 215C/425F. Line a large baking sheet with parchment paper. Cut the cauliflower into florets and toss with 2 tablespoons of olive oil and a pinch of salt. Spread evenly on the baking sheet. Bake for 25 to 35 minutes, tossing halfway. The cauliflower should be tender and caramelized on the edges.

In a large soup pot, warm the remaining 1 tablespoon olive oil over medium heat until shimmering. Add the onion and a pinch of salt. Cook, stirring occasionally, until the onion is softened (5-7 minutes).

Add the garlic and cook, stirring constantly, until fragrant, about 30 seconds, then add the broth.

Reserve a few of the roasted cauliflower florets for garnish. Transfer the remaining cauliflower to the soup pot. Bring the mixture to a simmer over medium-high heat, then reduce the heat as necessary to maintain a gentle simmer. Cook, stirring occasionally, for 15-20 minutes.

Remove the pot from the heat and let it cool for a few minutes. Then, carefully transfer the hot soup to a blender. Work in batches if necessary.

Add the butter and an additional pinch of salt, to taste. Blend until smooth.

Top individual bowls of soup with roasted cauliflower florets and a sprinkle of chopped dill.