Preheat the oven to 425°F. Toss the cauliflower florets and potato cubes with 2 tablespoons olive oil, turmeric, salt, and pepper on a baking sheet. Spread in a single layer and roast for 25 minutes until the edges turn golden.
While the vegetables roast, heat the remaining tablespoon of olive oil in a large pot over medium heat. Add the diced onion and cook for 5 minutes until softened.
Stir in the minced garlic and cook for 1 minute more. Pour in the vegetable broth and bring the mixture to a simmer.
Add the roasted cauliflower and potatoes to the pot. Simmer everything together for 10 minutes.
Remove the pot from the heat. Stir in the lemon juice. Use an immersion blender to puree the soup until smooth, or transfer to a countertop blender in batches.
Ladle into bowls and sprinkle with the chopped parsley before serving.