Meanwhile, finely chop fennel. Heat oil in a large pot over medium-high. Add fennel bulb, scallions, and sweet potatoes and cook, stirring occasionally, until fennel begins to soften, 3–4 minutes. Add cauliflower, and turmeric; toss to coat. Reduce heat to medium, add 3 Tbsp. water, and cook, covered, until cauliflower is just tender, 6–8 minutes. Uncover and stir in garam masala, half the cilantro; cook until fragrant, about 2 minutes.

Top with remaining cilantro