Bring a large pot of water to a boil. In a bowl, toss the cubed apples with the lemon juice. Add the cauliflower to the boiling water and cook until just tender, about 3 minutes. While that occurs, melt the butter in a large skillet set over medium heat. Add the apples and walnuts. When the cauliflower is tender, drain well, and add to the skillet.
Cook, stirring frequently, until the apples are tender, about 3 to 4 minutes. Season with salt and pepper, toss in the parsley, and serve.