1. Trim the spinach of coarse stems and blemished leaves, and wash it.
2. Heat the olive oil and garlic in a large (6- or 7-quart) pot over moderate heat, and cook until the garlic releases its fragrance—about 1 minute. Add the spinach by handfuls, stirring occasionally with a wooden spoon.
3. When all the spinach has been added, stir in the vinegar, salt, and pepper. Serve hot, at room temperature, or slightly chilled.