This is the recipe I tried from one of my cookbooks:
2 russet potatoes, cut into 1-inch pieces
½ head cauliflower, cut into florets
1 tablespoon olive oil
2-4 tablespoons gluten-free vegetable broth
½ teaspoon salt
Several pinches freshly ground black pepper
1. Place the potatoes in a 4-quart pot in enough cold water to submerge them, making sure there are about 4 inches of extra water on top for when you add the cauliflower. Bring the potatoes to a boil. Once boiling, add the cauliflower and lower the heat to a simmer. Let simmer for about 15 minutes, until the potatoes and cauliflower are tender.
2. Drain them in a colander, return them to the pot, and use a potato masher to mash them a bit. Add the olive oil, 2 tablespoons broth and the salt and pepper and mash a bit more. Use a fork to make sure all the seasonings are mixed well. If needed, add another 2 tablespoons of broth. Taste for salt.