Preheat oven to 400°F (205°C). Line a baking sheet with aluminum foil (optional). Arrange cauliflower on baking sheet, generously drizzle (or spray) with olive oil, season with salt and pepper. Roast for 30 minutes, until cooked and tender, turning halfway through.
Meanwhile, heat 2 tbsp olive oil over medium heat in a deep pot and cook onion for 2 minutes, until translucent. Add garlic and cook for 1 more minute, stirring frequently.
Add vegetable stock, paprika, cumin, salt, and pepper. Add cauliflower and simmer on medium for 10 minutes. Remove from heat and let it cool for 10 more minutes. At this point, you can reserve some of the cauliflower florets for garnishing the soup later.
Transfer to a blender (or alternatively use an immersion blender) and blend until smooth. Transfer soup back to a pot and set heat on low. Add in Boursin cheese, whisk until melted. Remove from heat and serve. Add lemon juice (optional).