Preheat oven to 400°F. Line a large baking sheet with parchment paper.
In a large bowl combine carrots,2 tablespoons of olive oil, and salt. Toss to coat evenly, then spread the carrots out in a single layer o the prepared baking sheet.
Roast the carrots for 35 to 40 minutes, flipping halfway through, until they are fork-tender and lightly caramelized.
In a Dutch oven over medium heat, add the remaining one tablespoon of olive oil. Once hot, add the chopped onion. Cook, stirring occasionally, until the onion is softened and translucent, about 5 to 7 minutes.
Add the minced garlic and cook for one more minute, stirring constantly.
Pour in the chicken broth and heavy cream. Stir to combine, scraping up any browned bits from the bottom of the pot.
Increase the heat to high and add the roasted carrots. Bring the mixture to a boil, then reduce the heat to medium-low and simmer for 15 minutes.
Turn off the heat. Add the unsalted butter and lemon juice. Using an immersion blender, carefully blend the soup directly in the pot until completely smooth. (You can use a countertop blender if you don't have an immersion blender, just be very careful as the liquid will be hot).
Season to taste with salt and pepper.
Ladle the soup into bowls and serve hot. Garnish with a drizzle of heavy cream and a sprinkle of chopped fresh parsley, if desired.