In a skillet over medium heat, melt butter with olive oil. Add red onion and cook until softened. Add diced squash and pour in vegetable stock. Simmer gently for 12–15 minutes, stirring occasionally, until the squash is very soft, and the liquid has mostly reduced. Mash with a fork or masher until spreadable. Stir in maple syrup, lemon thyme, parsley, tarragon, onion powder, lemon zest, salt and white pepper. Set aside to cool slightly.
Toss apples gently with olive oil, apple cider vinegar, parsley and lemon thyme. Reserve a small amount of the apple slices for garnish.
Spread herb mayo evenly over the bottom half of the sourdough. Spread a generous layer of mashed squash over the mayo. Lay 3 slices of Gruyère over the squash and torch until the cheese melts.
Layer the seasoned apple slices over the melted cheese. Spread a thin layer of herb mayo on the cut side of the top slice of bread, and then spread another layer of squash over the apples. Place the top bread slice on and press down gently. Top with the remaining 3 slices of Gruyère and torch to melt the cheese. Let rest for 5–10 minutes.
Trim and cut into 6–8 rectangular tea sandwiches. Garnish with reserved seasoned apples, snapdragons, marigolds and spray a light mist of edible gold before serving.