Heirloom Tomato, Zucchini, Caramelized Onion and Feta Galette

yellow onions, peeled and sliced

olive oil, plus more for drizzling

butter

refrigerator pie crusts (1 box)

egg white, beaten

Dijon mustard

heirloom tomatoes, sliced

zucchini, thinly sliced

feta cheese, crumbled

Sprigs of fresh thyme leaves

Kosher salt and freshly ground black pepper