Preheat oven to 400°F. Line a large baking sheet with parchment paper.
Place the carrots on the baking sheet. Drizzle with 2 tablespoons of olive oil, and sprinkle with kosher salt. Toss to coat and arrange carrots in a single layer.
Bake until fork tender and slightly caramelized, about 35-40 minutes, flipping halfway through.
To a Dutch oven over medium heat, add the remaining olive oil. Once hot, add onion and cook, stirring occasionally, until softened, about 5 to 7 minutes.
Add garlic. Cook one more minute.
Pour in the chicken broth and heavy cream, scraping up any browned bits from the bottom of the pot as you go.
Increase the heat to high and add the roasted carrots. Bring to a boil. Then, reduce the heat to medium-low, and simmer for 15 minutes.
Turn off the heat. Add the butter and lemon juice. Using an immersion blender, blend until smooth. Season to taste with salt and pepper.
Serve the soup hot, garnished with freshly chopped parsley and a drizzle of heavy cream.