Preheat a large sauté pan over medium heat and melt the butter and olive oil together. Add the sliced onions, season with salt and pepper and reduce the heat to simmer and slowly cook the onions for about 40-45 minutes, stirring occasionally until the onions are soft and caramelized. Set aside to cool.

Cut one of the refrigerator pie crusts into a roughly 7 X 9-inch square, so that you trim the rounded edges from the crust and discard or save for another use. Place the pie dough on one half of a large baking sheet lined with parchment paper. Lightly brush the bottom of the crust with the beaten egg white. Spread 1 tablespoon of the Dijon mustard on the crust, leaving a 1-inch border of plain pie crust all the way around the edge. Scatter half of the caramelized onions on the mustard then layer 2 rows of zucchini slices down the middle of the crust. Top with slices of tomato and sprinkle with thyme leaves and half of the feta cheese then drizzle with a little additional olive oil. Season with salt and pepper. Fold the long edges of the pie crust over and then the short edges over the zucchini slices. Repeat the process with the other crust.

Preheat the oven to 400 degrees F. Bake the galettes for 15 minutes or until the crust is golden and the vegetables are softened.

Slice and serve warm or at room temperature.