For the filling:
2 white onions, peeled and thinly sliced
2 tablespoons unsalted butter, divided
2 medium sweet potatoes, peeled and cut into 1⁄2 inch dice
1⁄2 tablespoon fresh sage, chopped
1 cup shredded raclette cheese
For the dough:
1 cup all-purpose flour
1 cup whole wheat flour
1⁄2 teaspoon salt
1⁄2 teaspoon fresh ground black pepper
1⁄3 cup water
2⁄3 cup olive oil
First begin caramelizing the onions, as this will take about 45 minutes. In a deep skillet over medium low heat, sauté the onions in 1 tablespoon of butter and a pinch of salt and sugar, stirring occasionally, until a deep golden brown. Allow to cool. In another skillet, sauté the sweet potatoes in the other tablespoon of butter, along with a pinch of salt, until soft. Allow to cool.
Meanwhile, make the dough. Sift together both flours. Add salt and pepper. Gradually incorporate the water, then the olive oil. Knead gently until combined. If the dough is too wet add more flour; if it’s too dry add more olive oil. Cover with plastic wrap and allow to rest for 30 minutes in the refrigerator.
When you’re ready to make the galette, preheat the oven to 375 degrees. On a sheet of parchment paper, roll out the dough to a 12-inch round approximately 1⁄8 inch thick. Trim excess dough. Layer the onions and sweet potatoes on the dough, leaving 2 inches around the edge. Top with the shredded raclette. Carefully fold up the edges of the dough, just covering the outer edge of the filling. Brush the dough with olive oil and slide the galette, still on the parchment paper, onto a baking sheet and bake for 40 to 45 minutes. Allow to sit 5 to 10 minutes before slicing into wedges.