Sourdough Stuffing with Sausage, Red Onion and Kale

unsalted butter, cubed, plus more for greasing

sweet Italian sausage, casings removed and meat crumbled 


medium red onions, cut into 1-inch wedges through the core


extra-virgin olive oil


Kosher salt

Black pepper


curly kale, leaves torn


large eggs


chicken stock or low-sodium broth


pounds sourdough bread, torn into 2-inch pieces


chopped parsley


tablespoon thyme leaves


crushed red pepper