Remove the stems from the swiss chard. Finely dice the chard stems and roughly chop the leaves. Finely dice the onion and mince the garlic.
In a skillet add both the olive oil and the butter over medium heat. Add the diced onion and sauté over medium heat until the onion starts to soften. Add the chard stems and continue to sauté until both the onions and stems are soft.
Once the onions and stems are soft add the minced garlic and sauté briefly. Add the chard leaves and toss in the pan until they are completely wilted. Season with kosher salt and pepper. Set the filling aside to cool to room temperature before filling the puff pastry.
Preheat the oven to 400°F / 205°C and line two large baking sheets with parchment paper.
To assemble the pinwheels, lightly dust the counter with flour and unroll one of the puff pastry sheets. Lightly spinkle with flour and gently roll the pastry out a bit. Roll out the pastry about an inch wider on each side making sure not to roll it too thin.
Drop half of the chard filling onto the pastry. Gently spread it out, leaving a half inch border along one of the narrow ends clear. The filling will be sparse and won't completely cover the pastry but spread it out as evenly as possible. Sprinkle half of the shredded Gruyère cheese over the filling.
Lightly moisten the clean edge of pastry with some water and begin to roll the pastry up from the opposite end. Roll the pastry a bit loosely so that there is room for the pastry to expand while cooking. Roll up to the moistened edge to seal the dough. Gently wrap the roll in cling film and store in the fridge while you prepare the second roll.
Repeat with the remaining puff pastry sheet, filling and Gruyère cheese.
Carefully slice the the rolls into half inch slices using a very sharp kinfe to prevent the roll from squishing. Lay the slices down on the parchment lined baking sheets leaving a little room between them to allow the pinwheels to expand.
Bake for 25 to 30 minutes, rotating the pans after 15 minutes to ensure even browning. Bake until the pinwheels are well puffed and evenly browned. Remove from the oven and let cool slightly before serving. Serve these pinwheels either warm or at room temperature.
Notes
Note: For the chard you will need approximately 2 cups of finely chopped stems and 4 cups of loosely packed chopped leaves. If the bunches of chard are small then use 2.