Preheat oven to 375. Peel the squash, then halve and scoop out the seeds. Cut into 1/2 inch pieces. Toss with 2 tbsp of olive oil and sprinkle with salt and pepper. Spread onto a prepared baking sheet and bake for 30 minutes until fork tender. Remove from oven when done and let sit at room temp.
While the squash is cooking, make the onions. Peel the onion and halve lengthwise. Then slice in thin half moon shapes. Give the onions a quick rinse under water. In a medium pan over low heat, add 1 tbsp butter and the remaining 1 tbsp olive oil. Let the butter melt down then add the onions and brown sugar. Let the onions cook for about 25 minutes, stirring occasionally to evenly brown. At the end, stir in the pinch of nutmeg. Take off of heat.
In a small pan over medium heat, melt the remaining 4 tbsp butter. Once melted, add in the sage leaves and cook until butter browns and sage leaves are crispy, about 4 minutes. Stirring occasionally. Remove sage leaves and set aside.
Roll out the pie crust. It's best to take it out of the fridge about 10 minutes before you plan on rolling it out. Place on parchment lined baking sheet. Assemble the galette. Brush about half of the browned sage butter on the pie crust. Leaving a 2-3 inch border around the edge, spoon in the butternut squash, then caramelized onions, then the shredded gouda. Pour the remaining browned sage butter on top. Fold the edges of the pie crust on top of the filling. The center will be open. Brush the edges of the crust with an egg wash if using and bake for 35-40 minutes until golden brown. Let sit at room temp for 5-8 minutes before serving. Best served nice and warm but also fantastic at room temp.