Preheat oven to 425°F (220°C). Cut cauliflower into 1.5-2 inch florets. Place on a baking sheet, drizzle with 2 tablespoons olive oil, season with salt and pepper, and toss to coat.
Roast cauliflower for 25-30 minutes, flipping once at halfway point, until edges are deep golden brown and centers are tender.
Heat remaining 1 tablespoon olive oil in a Dutch oven over medium heat. Sauté onion for 6-8 minutes until soft and translucent. Add garlic and ginger, cook for 60-90 seconds until fragrant.
Add curry powder, turmeric, and cayenne. Cook for 1-2 minutes, stirring constantly, until spices are fragrant and color deepens.
Add roasted cauliflower to the pot and stir to coat. Pour in vegetable broth, scraping up any browned bits. Bring to a boil, reduce heat to medium-low, cover, and simmer for 10 minutes.
Remove from heat and cool for 2-3 minutes. Blend with an immersion blender directly in the pot until completely smooth and creamy, about 60-90 seconds.
Return to medium-low heat. Stir in coconut milk and lemon juice. Season with salt and pepper to taste. Heat gently for 3-5 minutes, but do not boil.
Ladle into bowls and garnish with a swirl of coconut milk, fresh cilantro, red pepper flakes, and toasted pumpkin seeds if desired. Serve immediately.