Cook the bacon in a large skillet over medium-high heat until crisp, about 5 minutes. Add the garlic, thyme and red pepper flakes and cook until just fragrant, about 30 seconds. Add the spinach and some salt and pepper, and cook, tossing, until the spinach is wilted, about 2 minutes. Add the balsamic vinegar and stir to combine. Serve hot or at room temperature.