Preheat the oven to 400°F (200°C).

Cut potatoes into 1-inch cubes. Trim broccoli into florets twice the size of the potatoes. Slice the onion into half-moons.

Combine potatoes, broccoli, and onion on a sheet pan. Drizzle with olive oil, salt, and pepper. Roast for 30 minutes, flipping once after 20 minutes.

Bring half of the chicken broth to a boil. Blanch broccoli rapin for 15 seconds. Set aside.

Remove sheet pan from oven. Add remaining chicken broth to the pan and scrape up any browned bits.

Transfer roasted vegetables and liquid to a food processor or blender. Blend until smooth, adding more broth if needed.

Pour soup into a pot and heat over low. Stir in crumbled Stilton and crème fraîche until dissolved.

Serve in bowls topped with crumbled Stilton and chopped broccoli rapin.