Preheat the oven to 450°F (230°C) and set two racks in the oven, spaced apart evenly.

Toss the potatoes in ½ teaspoon (0.5 ml) salt and 1 tablespoon (15 ml) oil. Spread them on a baking sheet, cut side down, and roast for 20 to 25 minutes, until browned on the underside.

Toss the cauliflower in ½ teaspoon (0.5 ml) salt and 1 tablespoon (15 ml) oil. Spread the florets on a baking sheet, cut side down, and roast for about 15 minutes, until browned on the underside.

While the vegetables roast, make the curry sauce. Place ¼ cup (185 ml) coconut milk in a medium pot and bring to a boil over medium-high heat. Stir in the curry, mixing well, then turn the heat down to medium and stir constantly until the mixture is very thick and the coconut oil separates from the paste, about 5 minutes.

Stir in the remaining coconut milk, 1 tablespoon (15 ml) fish sauce, 1 tablespoon (15 ml) tamarind paste, and sugar. Simmer over low heat for at least 5 minutes, until the vegetables are done roasting.

Add the roasted vegetables and the onions to the curry sauce, then add just enough of the stock to submerge everything. Simmer the vegetables for about 5 minutes to allow them to absorb the curry flavor.

Stir in the tomatoes and turn off the heat, allowing them to soften in the residual heat for a few minutes. Taste and adjust the seasoning, adding more fish sauce, tamarind paste, or sugar as needed.

Serve with jasmine rice.