Cut butter into small cubes and place in the fridge to firm up while you get the rest of your ingredients together.
In a medium sized bowl, whisk together the flour, hazelnut flour, sugar, and salt. Add in the cold butter and use your hands to press it into the flour mixture until you get quarter sized crumbs.
Add cold water and combine until dough forms a nice ball but is still slightly sticky.
Take the formed ball of dough and shape into slightly flattened disk.
Wrap the disk in plastic wrap and chill in the fridge for at least 30 minutes. This is important to keep the butter from melting so that the crust will be nice and flaky. The chilled dough is also much easier to work with for rolling out.
You can also make the pie dough up to 2 days in advance and store in the fridge until ready to use!
Preheat your oven to 400°F.
Gently toss together the sliced plums, lemon juice, almond extract, sugar, and cornstarch until evenly coated.
Lightly flour a clean surface. Roll out dough to about ¼ inch thick and about 12 inches across. If you want a cleaner looking galette, you can use a knife to trim the edges of dough to make a perfect circle but it's not necessary.
Place the dough on a parchment lined baking sheet.
Arrange the plums on your pie dough, leaving about an inch around the edges. [I love doing a pretty flower design, but you can just pile them on if you prefer.]
Gently fold over the edges of the dough overlapping as you go.
Brush the edges lightly with egg wash and sprinkle with a little extra sugar, if desired.
Bake at 400°F for 20-25 minutes or until the filling is bubbling and the crust is golden brown.
Let cool for at least 10 minutes, then sprinkle with fresh thyme and toasted hazelnuts and serve with vanilla ice cream or homemade whipped cream.
While this rustic plum galette is delicious warm, the filling will set more as it cools.
Notes
This galette is best eaten the same day, but can be stored in the fridge for 2-3 days. You can serve it cold or warm it up in the oven at 350°F for about 5 minutes to crisp up the crust.