Bring the phyllo dough to room temperature. To prevent the dough from drying out, place the phyllo between damp towels.
Preheat the oven to 375°F. Line a baking sheet with parchment or a silicone baking mat.
Brush the cut side of the butternut squash with ½ tablespoon of the oil and season with a pinch of salt. (I like to roast both halves of the squash together and save the other half for another recipe like soup or risotto!) Place the squash cut-side down on the prepared baking sheet. Roast for 35-45 minutes until the squash is very tender when poked with the tip of a sharp knife.
Meanwhile, heat 2 tablespoons of the oil in a large skillet over medium heat. When the oil is hot and shimmering, add the onion and a small pinch of salt. Cook for about 10 minutes or until very soft and very lightly browned (you just want a little color on the onion, not full browning). Add the apple cider vinegar and maple syrup and cook for about 8 to 10 minutes until the liquid has reduced and the onions have a “jammy” consistency. (Be very careful not to burn this mixture!) Stir in the sage and cook for 30 seconds, then transfer the onion mixture to a medium bowl.
Scoop the flesh from the roasted squash into the bowl with the onions (you should have a little less than 2 cups of squash). Add the ricotta and a generous pinch of salt and black pepper. Fold all the ingredients for the filling together until combined. Taste and add more salt and pepper as needed.
Time to assemble the turnovers! Place one sheet of phyllo dough on a cutting board. Lightly brush with a little bit of the remaining oil. Layer with a second sheet of phyllo and brush lightly with oil again. Slice the phyllo dough lengthwise into three even strips. Add one tablespoon of filling to the end of each phyllo strip leaving a little space around it. Fold the dough over at a 45° angle to create a triangle, then continue to fold the triangle back and forth onto itself until a turnover is formed. If you have a little extra dough leftover when you’ve finished forming triangles, just trim it with your knife. Repeat this process with the remaining phyllo dough and filling until all your turnovers have been formed. (You may or may not need extra olive oil.)
Place the finished turnovers on a baking sheet lined with parchment or a silicone baking mat. (You will likely need two baking sheets.) Brush the tops with a tiny bit more olive oil and season with a little flaky salt (or kosher salt) and black pepper.
Bake at 375° for 14-17 minutes (keep a close eye on them!) until the turnovers are golden brown. Serve warm.