Preheat the oven to 425°F and line a baking sheet with parchment paper. Wrap the thawed spinach in a clean kitchen towel and squeeze out as much liquid as you possibly can, until it is very dry and crumbly.
Blend the cottage cheese until completely smooth and refrigerate it. The smooth cottage cheese adds a gentle tang that pairs perfectly with the feta and keeps the crumb soft and moist.
Whisk the flour, baking powder, baking soda, salt and pepper in a large bowl. Grate the frozen butter into the flour on the large holes of a box grater and toss to coat.
Add the spinach, feta, green onions, dill and lemon zest, tossing and breaking up any spinach clumps so the greens are evenly spread through the flour.
Pour in the blended cottage cheese and stir with a fork until a shaggy dough forms, adding milk 1 tablespoon at a time only if dry flour remains. Turn it onto a floured surface, fold it over on itself twice and pat it into an 8-inch square about 1 inch thick.
Cut the dough into 9 squares with a bench scraper and place them 2 inches apart on the baking sheet. Brush the tops with beaten egg and sprinkle with sesame seeds.
Bake for 17-20 minutes, until deep golden and crisp at the edges. Cool for 5 minutes and serve warm.