Preheat oven to 350 degrees F. Place whole chicken breasts in boiling water for 10 minutes or until cooked.
In the meantime, saute the chopped onions in butter until soft. Add cream cheese to onion mixture and stir until smooth. Remove cooked chicken and cut into small chunks. Add chicken to cream cheese mixture and stir.
Unfold the packaged croissants to form 4 rectangles and place on a greased cookie sheet. Repeat to make 4 more rectangles for the top. Spoon the chicken mixture in the middle of the croissant square leaving room around the edges and place another croissant square on top. Pinch the croissant square edges together and tops closed to prevent mixture from running out. Brush tops with egg white wash (optional).
Bake in 350 degrees F oven for 12-15 minutes until croissant crust is golden brown.