Wash the chard, remove the stems from the attachment of the leaves and remove any stringy fibrous filaments. These are not great to eat.
Finely slice the stems and set aside.
Continue to slice the leaves more coarsely, and set aside too.
Chop the chili pepper, if used, chop the garlic and cut the olives into halves or thirds, depending on your preference. If you don't like garlic, you can crush the cloves and remove them after browning. This will give a more mild flavour.
In a large pan (wok type) add 1 tablespoon of oil, chilli and minced garlic. Leave to fry for about a minute, then add the chopped chard stalks.
Add a pinch of salt, pepper and sauté over medium-high heat for about 3-5 minutes.
Add the leaves of the Swiss chard and mix everything well. Then pour in the water, lower the heat, close with the lid and leave it for 10-15 minutes (depending on your taste
Turn off the heat and add the olives, capers and almond flakes. Leave to cool for a while.
Meanwhile preheat the oven to 200° C (390° F).
Line a 20x30 pan or baking sheet with parchment paper, then lay the first layer of filo dough on it. Brush with olive oil and add another sheet of pasta. Continue until all the dough sheets are finished, making sure to spray each of the layers with a little oil.
Arrange the sautéed chard filling in a rectangle in the centre of the pastry, leaving about 3 cm from the edges and taking care to drain any remaining cooking water. If you do not drain it, the pastry will become soggy.
Fold the 4 sides of dough over the filling, then form a roll by rolling the strudel on itself from the widest part.
Finish with a final brush of oil, decorate with sesame or poppy seeds, and bake for the first 15 minutes on the low shelf of the oven, then move the strudel to the center of the oven and continue cooking for another 15-20 minutes.
Remove the strudel from the oven and let it cool for at least 30 minutes before serving.