For the galette: Preheat the oven to 375 degrees F.
In a large saute pan, add the olive oil. Add the sweet potatoes and saute for about 5 minutes over medium heat. Add the Swiss chard stems, garlic and onion. Cook, stirring frequently, until the onions are tender and translucent, 3 to 4 minutes. Add the Swiss chard leaves and continue to cook until the leaves are tender, 2 to 3 minutes. Remove from the heat and let cool.
Once cooled, add the walnuts, olives, feta and sun-dried tomatoes to the mixture, then toss until the ingredients are evenly distributed. Season with salt and pepper.
Line an 18-by-13-inch baking sheet with parchment paper. Place a sheet of phyllo on the baking sheet so the corner of the phyllo sheet is aligned with a corner of the baking sheet. Brush with butter. Place another piece of phyllo overlapping the first sheet, making sure the corner of the phyllo sheet is aligned with another corner of the baking sheet. Repeat with 2 more sheets of phyllo, brushing with butter in between so the entire baking sheet is covered with a layer of phyllo. This is the first layer consisting of 4 sheets of phyllo. Repeat twice more with the remaining 8 sheets of phyllo so you have 3 layers total. Once all the sheets are stacked, spread the filling out into a rectangle, just smaller than a 9-by-13-inch pan. Fold over the dough so it just begins to cover the filling all around the edges. Bake until the crust is golden brown, 15 to 20 minutes.
For the sauce: In a small saucepot over medium heat, combine the olive oil, crushed red pepper flakes and garlic. Cook until the garlic begins to toast and becomes golden brown, about 2 minutes. Add the red wine vinegar to stop the garlic from continuing to brown. Reduce by half and add the honey. Bring to a boil, then remove the mixture from the heat and let cool until thickened slightly. Add the walnuts and olives. Season with salt and pepper if needed. Let sit at room temperature for 15 minutes before serving.
Serve the galette with the sauce on the side.