Heat oil in skillet over medium heat. Saute garlic and red onion for 2 minutes or until tender. Add spinach and saute for 3 minutes or until moisture is evaporated. Squeeze to remove any additional moisture.
Preheat oven to 400° F. Line a large baking sheet with parchment paper. Divide dough into 4 equal pieces. On lightly floured surface roll each into 8-inch rounds. Transfer rounds to large baking sheet. On half of each circle and leaving 1-inch around edge, spread 1/4 cup of pizza sauce.
Shred chicken if in chunk form. In a bowl, mix the canned chicken, spinach mixture, red peppers, artichokes, olive and feta. Divide equally on half of each round over the pizza sauce. Top each with ¼ cup mozzarella cheese. Gently fold dough over the filling and seal the edges with a fork. Cut 2 slits on top. Transfer panzerotti onto prepared baking sheet and brush tops with milk and sprinkle with parmesan cheese.
Bake for about 25 minutes or until crust is golden brown. Let stand for 5 minutes.