Heat oven to 350°F. Place spinach in a large covered saucepan with the water still clinging to its washed leaves and cook over medium heat for 3 to 4 minutes until wilted, turning once.
Transfer spinach to a colander and cool for 5 minutes, then squeeze firmly in a clean towel and chop finely.
Heat olive oil in a skillet over medium heat and cook scallions for 2 minutes. Add garlic for 30 seconds, then remove from heat.
Mix drained spinach, scallion mixture, feta, yogurt, dill and pepper. Divide the filling over the croissant bottoms and close with the tops.
Arrange sandwiches on a sheet pan and tent loosely with foil. Bake for 8 minutes, then uncover for 3 to 4 minutes until the filling is hot and the pastry edges are crisp.
Rest for 2 minutes before serving so the soft filling settles inside the croissants.