In the bowl of a food processor fitted with an s-blade combine the spelt flour, all purpose flour, salt, and cubed butter. Pulse the food processor several times, until the butter is roughly the size of grains of rice. With the food processor running add the ice water one tablespoon at a time. You want the pastry to be starting to pull together, but it should still be quite crumbly at this point. Stop the food processor and see whether it holds together if you squeeze it in your hand. If yes, you're done. If no, add one more tablespoon of water.
Tip the pastry out onto the counter top and bring it together with your hands. Form it into a disk and wrap with plastic wrap. Refrigerate the pastry for at least 30 minutes, or overnight.
To make the galette
Slice the tip off of the onions (while leaving the root intact), slice in half, and peel the skin off. Place the cut-side down on a cutting board, and use a sharp knife to slice the onions into thin slices.
Heat the oil in a large heavy-bottomed skillet (preferably cast iron) over medium heat. Add the onions and allow to cook without stirring for 5-10 minutes.
Every five minutes or so, stir the onions and then re-arrange into a flat layer in the skillet. It should take about 30 minutes for them to become very soft and a deep golden brown, and reduce to about ⅓ the original size. Watch that they don't burn - you want deep brown, not black.
Remove the onions from the heat, add the thyme and season with salt and pepper. Set aside to cool.
Preheat your oven to 375°F. Line a baking sheet with parchment paper.
Roll the chilled pastry out into a thin disk, about ⅙ of an inch thick. Don't worry about what shape the pastry becomes as you roll it out, this is supposed to have a rustic look.
Carefully transfer the pastry to the prepared baking sheet.
Spoon the cooled onions over the center of the pastry, and spread out into an even layer, leaving a few inches of pastry all around to be folded in.
Crumble the goat cheese and evenly distribute over the onions.
Carefully fold the edges of the pastry in, so that they envelop about half of the onions, but the center remains open.
Place the galette into the oven and bake for 35-40 minutes, until the pastry is golden around the edges, and the onions are bubbling.
Remove from the oven and let cool for 10 minutes before slicing.
Leftover pastry freezes really well, so make a double batch and freeze for the next time you make a galette.
Mix up the cheeses you use here. Gruyere is lovely, as is a crumbled aged blue.