3 1/2 lb Yukon Gold potatoes, peeled and cut into 1" pieces
1 1/2 lb cremini or white mushrooms, halved
1 T butter
1/2 c buttermilk
1 egg plus 1 egg white, beaten
1 1/4 t salt, divided
1 T extra-virgin olive oil
3/4 c chopped shallots
3 cloves garlic, finely chopped
4 c trimmed and finely chopped chard or spinach
1 c mushroom broth or reduced-sodium beef broth
2 T flour
1/2 t pepper
1/2 t rosemary
1 c Parmesan, divided