Without peeling it, cut the yam into 3/4 inch cubes. Slice the green onions including the white tops & the fennel stems. Cut the cauliflower into 1 inch florets & chop the leaves & stems into smaller pieces.
In a Dutch oven or a heavy duty pot with a lid, heat up the coconut oil & pop the mustard seeds, then add the asafoetida. Add the yam, stir, lower the heat & cover the pot for 3-4 minutes, stirring occasionally or until the yam is almost cooked through. Add the green onions & sliced fennel stems, raise the heat to medium & cook for another minute or two. Then add the cauliflower, turmeric, serrano & salt. Mix together & cook for 3-4 minutes. Then lower the heat & cook covered for another 2-3 minutes, or until the cauliflower is cooked (tender but not soft).
Turn the heat off, stir in the garam masala, toasted cashew pieces, cilantro & fennel fronds.