In a large pan (I used a large wok) heat 1 tablespoon of the oil over medium high heat. Add the mushrooms and allow them to brown on one side. Stir and brown on the other side. Remove to a plate.
Lower the heat to medium, add the second tablespoon of oil and the onion. Fry until soft and transparent and then add the diced Swiss chard stems. Fry, stirring often, until tender. Add the garlic and fry for 30 seconds or until soft and fragrant.
Lower the heat to medium low and add the chopped Swiss chard leaves. Toss and stir as they begin to wilt. If your pan isn’t big enough, you might need to add the leaves in batches. Allow the leaves to cook, stirring occasionally, until very tender – about 10 minutes.
Meanwhile, preheat the oven to 400 F / 200 C or as indicated on the package of puff pastry.
Cut both sheets of puff pastry into 9 squares or rectangles. The brand of puff pastry I use comes pre-rolled and measures 13 x 10 inches (32.5 x 25.5 cm). If yours is smaller than this you can gently roll it out on a lightly floured counter.
When the chard is tender, turn off the heat, stir the mushrooms back in and season with salt and pepper. Let cool for a couple of minutes until you can touch it without burning your fingers.
Spoon the filling over half of the puff pastry pieces. I tried to pack it together with my fingers in order to have well-filled pastries. Top each with a second piece of puff pastry and seal the edges with a fork.
Bake for 20 – 30 minutes until puffy and golden brown or for the length of time indicated on the package of puff pastry.