Place the cauliflower in a large, microwave-safe dish; cook in microwave on High for 3 minutes. Transfer the cauliflower to a bowl and set aside. Put the potatoes in the dish and cook in the microwave on High for 4 minutes. Pour into the bowl with the cauliflower.
Heat the olive oil and cumin seeds in a large frypan over medium-high heat until the cumin swells and turns golden brown; stir the onions into the oil and cook about 3 minutes.
Add the tomatoes and cook and stir another 3 minutes. Fold the cauliflower and potatoes into the mixture. Season with the curry powder and salt. Continue cooking until completely hot, 3 to 5 minutes. Serve hot.