Step 1Using a sharp knife or scissors, cut the stem out of each of the kale leaves. Wash the leaves well and spin dry. Stack them and cut crosswise into one-half-inch strips.
Step 2Heat the oil in a large, deep skillet that has a lid over medium-high heat. Add the garlic and sprinkle with about a teaspoon of salt. Cook, stirring, until the garlic softens, 2 to 3 minutes. Add the kale a large handful at a time, stirring constantly until the leaves wilt before adding more. When all the leaves are in the pan, cook, stirring constantly for 2 to 3 minutes. Add a sprinkling of pepper flakes and the stock or water, stir well and reduce the heat to low. Cover the pan and cook the kale, stirring occasionally, until it is cooked, 20 to 25 minutes if you like it chewy, longer for very soft. (The dish can be made up to several hours ahead to this point.)
Step 3Uncover the pan and raise the heat to cook away most of the liquid remaining in the pan. Taste and add more salt if needed. Drizzle with truffle oil, if desired, and serve.