In stand mixer, combine flour, salt and sugar.

Add butter and mix on low for 10 seconds until all butter is coated in flour.

Scrape bowl and mix on low for 30 seconds until dough begins to break up and crumble.

Slowly add water as dough is mixing; stop once dough is shaggy.

Divide dough in half, pat each half into a disk, wrap in plastic and chill for at least 1 1/2 hours.

Line baking sheet with parchment paper. Unwrap dough and place on baking sheet.

Using rolling pin, roll each piece of dough to ¼-inch thickness.

Cut each piece into five roughly 6x5-inch rectangles, making 10 rectangles. (Note: A 5-wheel accordion cutter or pastry wheel both work well here.)

Preheat oven to 425 degrees F.

Over mixing bowl, place strainer lined with cheesecloth.

In sauté pan over medium-low heat, add spinach and oil and sauté about 5 minutes.

Transfer spinach to strainer to drain; let cool to room temperature.

Using cheesecloth, wring out liquid from spinach; transfer to mixing bowl and set aside.

In sauté pan, cook shallots and garlic until they sweat. (Note: Do not brown.) Let cool and add to bowl with spinach.

On baking sheet, arrange mushrooms, season with cumin, paprika, salt and pepper and bake until golden. Let cool. (Note: Leave oven on.)

In food processor, pulse together mint, parsley and oregano until fine, then transfer into spinach mixture.

In food processor, pulse mushrooms. Use cheesecloth to strain a bit and add to spinach mixture.

Add feta and stir to combine all filling ingredients.

In small bowl, beat egg with a few drops of water to make egg wash.

Place spoonful of filling in center of five dough rectangles.

Smooth out filling, allowing for ¼-inch dough border. Brush edges of each with egg wash. Position remaining, unfilled rectangles over each and press down firmly with fingers around edges.

Using fork tines, crimp edges of each tart.

Glaze top of each tart with egg wash; use fork to pierce tops several times; sprinkle with sesame seeds and salt flakes.

Bake 15 minutes or until golden. Let cool slightly before eating.