Wash the destalked spinach in several changes of cold water, then allow it to drain in order to remove as much of the water as possible.
Place a wide, low-sided, heavy saucepan or frying pan on the heat and add the olive oil. Immediately add the peeled thinly sliced garlic, and cook gently until the olive oil gets hot and the garlic colours slightly. Add the crushed dried chilli or the chilli flakes, whichever you are using, and cook for a few seconds – the garlic should now be golden.
Increase the heat and add the drained spinach, being careful because it will hiss and spit a bit. Cook the spinach, turning it constantly, then season with a little salt and freshly ground black pepper.
Add the lemon juice and zest, and stir them in. Pull the pan off the heat and taste. Correct the seasoning and serve the spinach immediately in a hot dish. The spinach can also be removed from the pan when it is cooked, spread out flat to cool, and served later at room temperature.