Heat the olive oil in a skillet over medium heat.
Cook the chopped onion and minced garlic for about 3 to 4 minutes, until soft.
Transfer the onion/garlic mixture to a bowl. Stir in the cottage cheese, mozzarella cheese, thawed spinach, egg, egg white, flour, dill, and black pepper.
Cut the 12 sheets of phyllo pastry into thirds (36 ⅓-sized sheets). Cover with plastic wrap and a damp towel, as phyllo pastry dries very quickly!
Take 1 piece of phyllo pastry, spritz it lightly with nonstick spray, and top with a second piece.
Spoon about 1 tablespoon of the filling mixture into the bottom corner of the phyllo rectangle. Fold the end of the rectangle over the filling to form a triangle. Keep folding the triangle over itself until all the pastry has been folded in.
Repeat 18 times. You have 36 ⅓-size rectangles of phyllo pastry. Spray 1 rectangle with nonstick, lay another on top (2 pastry rectangles per serving), spoon in a bit of filling, and fold end end of the rectangle over. You should end up with 18 little spanakopita triangles.
Preheat the air fryer to 375 °F
Spray both sides of the spanakopita triangles lightly with nonstick spray.
Air fry for about 6 minutes: 3 minutes on one side, and 3 minutes on the other side. The spanakopita triangles are done when they are lightly golden-brown.