Line a baking sheet (or two, if necessary) with aluminum foil, and place it on the rack. Preheat the oven to 475°F.  Note, you can also roast the potatoes and cauliflower in the air fryer if you prefer, to save time.

Toss the cauliflower florets and diced potatoes together in a large bowl with 3 tablespoons of oil, the cumin seeds, and 1/4 teaspoon of salt. 

Spread in the hot baking pan and roast, stirring occasionally, until the cauliflower is tender and charred in spots and potatoes are crispy and just tender, about 20 minutes.  In my air fryer on Roast at 400 F, it took 10-15 minutes.

About halfway through the vegetables roasting, heat a heavy 12-inch skillet over medium heat (erring on the side of lower).  Cook the onion, garlic, and ginger in the remaining 2 tablespoons of oil, stirring occasionally, until very soft and turning golden brown.  This should take about 8 to 10 minutes...I like to get mine pretty brown, but you don't want them to burn (it makes the flavor meh).

Add the ground cumin, coriander, turmeric, cayenne, and remaining 1/2 teaspoon of salt and cook, stirring constantly, for 1-2 minutes.

Stir in the water, scraping up any brown bits from the bottom of skillet, then stir in the roasted cauliflower and potatoes.

Cook, covered, stirring occasionally, for 5 minutes. Taste and add more salt if needed. Squeeze lemon over it when serving, if desired.