In a food processor, combine spelt flour and salt, pulsing until well mixed. Add chilled butter and pulse until the mixture resembles coarse crumbs, about 20 seconds. Gradually drizzle in apple cider vinegar and ice water, mixing until the dough appears shaggy. Gather the dough into a disc, wrap in plastic, and refrigerate for at least 2 hours.

Slice onions into half-moons. In a skillet, heat olive oil or butter over medium-low heat and add onions. Cook slowly, stirring occasionally, for about 20-25 minutes until they turn golden brown and very soft.

After the dough has chilled, place it on a lightly floured piece of parchment paper. Roll the dough into a circle about 1/4 inch thick, leaving the edges slightly uneven. Chill the rolled dough for another 10 minutes.

Spread a layer of goat cheese over the center of the dough, leaving a 2-inch border. Top with caramelized onions and layer fresh asparagus on top. Fold the edges of the dough over the filling, overlapping as necessary to create a freeform shape.

Preheat your oven to 400°F (200°C). Brush the exposed edges of the dough with egg wash. Optionally, sprinkle seasoning over the crust. Bake for 45-50 minutes until the crust is golden brown.

Once baked, let the galette cool for a few minutes on the baking sheet. Slice into wedges and serve warm.