Gather and measure ingredients for galette crust. Chop the butternut squash into 1/2" pieces. Wash the mushrooms and thinly slice. Thinly slice the red onion. Mince the garlic. Chop the sage and remove leaves from thyme.
Heat a medium sized skillet on the stove using medium heat. Add 2 tbsp of the olive oil. When oil is hot, add minced garlic and stir for about 20 seconds. Add the onions, mushrooms, 1/4 tsp of salt, and a little ground pepper. Cook for 20-25, minutes stirring occasionally. Once the water from the mushrooms has cooked out, add the fresh thyme.
Place the butternut squash in a bowl and add the remaining 1 tbsp of olive oil, sage and remaining 1/4 tsp salt. Stir to coat. Place on a parchment lined sheet pan and roast in the oven for 20-25 minutes until soft.
While monitoring and still occasionally stirring the mushrooms, make the galette dough. See link above for instructions on combining ingredients for the dough (you can eliminate the coconut sugar). Once the dough is made, roll it out between two pieces of parchment, creating a circle about 12" in diameter. Remove the top piece of parchment and discard. Slide the rolled out dough onto a flat sheet pan. Make the egg wash by whisking an egg with 1 tsp of water and brush onto the rolled out dough. (This helps prevent a soggy crust.) Keep the egg wash for brushing at the end.
When the vegetables are finished cooking, remove the butternut squash from the oven and turn off the heat for the mushrooms and onion mix.
Transfer the butternut squash onto the rolled out dough. Spread evenly throughout, keeping about a 2" border around the edges. Add the mushroom and onion mixture over the top of the squash and spread out.
Fold over one section of the galette border. It's easiest to do this by placing your hand under a portion of the parchment paper and gently folding over. Leave the center uncovered. Continue folding over sections of the edges, slightly overlapping with the previous fold. Continue folding sections until the entire galette is shaped. Because the crust lacks gluten, it will likely break in places. Simply mend broken pieces with your fingers, using a little water if necessary. It is fine having some broken pieces on the top as the baked finished product will look elegantly rustic. Brush the dough with more of the egg wash.
Place the galette in the oven (still at 400°F) and bake for about 22-26 minutes. The crust should be lightly browned. Remove from the oven and let rest for 5 minutes.
Cut the galette into 4 pieces and serve while hot. Garnish with additional herbs if desired and add salt and pepper to taste.
Notes
This paleo butternut squash mushroom galette can be a couple hours ahead of time and placed in the refrigerator until baking.