Heat the olive oil over medium heat in a large skillet. Add the garlic and the chili flakes and simmer until the garlic has softened, about a minute.
Add the kale to the skillet in batches as it wilts down so you can ensure even coverage with the oil and chili flakes (it will also wilt down faster if you add it in smaller batches.) When all of the kale has been wilted down in the skillet, transfer it to a bowl; set it aside.
Add the broth to the same skillet and bring to a simmer. Then, add the beans and let the broth continue to simmer to warm the beans through. When the beans have absorbed most of the broth (this should take about 5 minutes), add the greens back to the pan, and then add the green olives; give everything a good stir.
Empty the can of sardines into the skillet with all of its contents (oil included); stir.
Add the lemon zest and lemon juice to the dish, and then hit it with some cracked black pepper and more salt if necessary.
Serve with toasted slices of baguette!