Place freekeh and 1¾ cups water in a medium pot and bring to a boil over medium-high heat. Cover, reduce the heat until the water barely simmers, and cook freekeh until tender and the water has been absorbed, about 20 to 25 minutes. Spoon into a bowl and then cool to room temperature.
Pour ½-inch of water into a large pot and bring to a boil. Add the spinach and cook and stir until it just wilts, about 3 to 4 minutes. Set a strainer in the sink and pour and drain the spinach in it. Rinse spinach with cold water to cool. Firmly press the spinach and squeeze out as much water as you can. Finely chop spinach and add it, and all remaining ingredients, to the freekeh and mix well to combine.
Preheat the oven to 375°F. Line a large baking sheet with parchment paper.
Set one sheet of phyllo on a work surface, with the widest side parallel to you, and brush lightly with melted butter. Top with another sheet and brush it with butter. Repeat these steps until all sheets are used.
Spoon on the freekeh mixture and shape it into a long, narrow, compacted rectangle along the bottom of the pastry nearest you. Fold over the sides of the pastry, about 2 inches on either side, and then carefully roll the filling into the pastry.
Brush top of strudel with a little melted butter. Make 6 shallow, diagonal cuts, about 2 inches apart, into the top of the pastry. Bake for 30 to 35 minutes, until puffed and golden. Allow strudel rest 5 minutes. Slice and serve with the tzatziki sauce for dipping.
Note: Boxes of freekeh are sold in our ethnic foods aisle.