In the bowl of a stand mixer fitted with a dough hook attachment, combine all ingredients. Mix 5 minutes on low speed, or until dough comes together. Increase speed and mix additional 7 minutes. Let sit 10 minutes. Transfer dough to a lightly floured work surface. Using a rolling pin, shape dough into a rectangle. Cover with plastic wrap and refrigerate overnight.

In a sauté pan over medium flame, heat oil. Add garlic, spinach, and herbs. Sauté 10 minutes. Strain mixture, squeezing out as much liquid as possible from the spinach. Fold in feta, then transfer to an airtight container and refrigerate.

Place Dough on a work surface. Place butter sheet on one half of Dough, then fold the other half over top to enclose butter in Dough. Using a rolling pin, roll out Dough. Fold in half, then fold in half again. Cover Dough with plastic wrap and refrigerate 1 hour. Transfer Dough to a work surface and roll out into a ¼-inch-thick rectangle. Spread Filling evenly over top. Fold one side to the center. Take the other side and fold the remaining half over so it rests on top of the first side. Cut into 2½-inch-wide portions. Transfer croissants, seam-side-down, to a parchment-lined sheet tray. Let proof 90 minutes.

Heat oven to 375°F. Sprinkle Croissants with sesame seeds and dried herbs. Bake 20 minutes, or until deep golden brown. Let cool, then serve.