Preheat oven to 450°F. Line a baking sheet with parchment paper
Place 4 c cauliflower in a large bowl and coat with cooking spray; toss with 2 tsp curry powder and 1/2 tsp salt. Spread cauliflower on prepared pan; roast, stirring once halfway through, 30 minutes.
While cauliflower cooks, heat oil in a large soup pot. Add onion and remaining 1/2 tsp salt; cook, stirring often, until slightly softened, 5 minutes. Add garlic, ginger, mustard seed, cumin seed and remaining 1/2 tsp curry powder (or more to taste); cook, stirring frequently, 1 minute.
Add remaining 4 c raw cauliflower florets and broth to pot; increase heat to high and bring to a boil over high heat. Reduce heat to medium-low; simmer, uncovered, until cauliflower is extremely soft, 15-20 minutes.
Puree soup in pot using an immersion blender (or puree in batches in a countertop blender); stir in tomatoes and green beans. Cook, uncovered, until green beans are tender, 5-7 minutes. Stir in lime juice and roasted cauliflower; sprinkle with basil.
Notes