Preheat the oven to 425°F (220°C).

In a bowl, toss together the chopped onion, cauliflower florets, butternut squash cubes, 1 1/2 tbsp olive oil, rosemary, salt, and pepper until evenly coated.

Spread the vegetables on a parchment paper-covered baking sheet.

Roast in the preheated oven for 45 to 60 minutes, or until the squash is tender, turning the vegetables after 30 minutes.

In a large saucepan, heat the remaining 1 1/2 tbsp olive oil over medium heat.

Add the minced garlic and cook for 1 minute, stirring constantly.

Add the roasted vegetables (reserving some roasted cauliflower for garnish, if desired) and vegetable broth to the saucepan.

Bring the mixture to a boil, then cover and reduce heat to low.

Simmer for 15 minutes.

Using a blender or food processor, puree the soup until smooth.

Return the pureed soup to the saucepan.

Stir in the milk and warm over medium heat without boiling.

Add more milk for a smoother consistency, if desired.

Serve the soup hot, garnished with finely chopped parsley, roasted cauliflower pieces, and a drizzle of extra-virgin olive oil.