Preheat oven to 400 degrees F. Spray a popover pan with cooking spray and set aside.
Place the cubed croissants on a baking sheet and toast until golden brown and somewhat dried out, about 15 to 20 minutes.
Heat oil in a a large skillet over medium high heat. Cook the ham until just starting to crisp, about 6-8 minutes.
Add the onion, celery and garlic and cook until softened, about 5 to 7 minutes. Add in the spinach and cook until wilted down. Season with salt, pepper and rosemary.
In a large bowl, mix to combine the croissant cubes, sauteed ham and vegetable mixture, chicken stock, eggs, scallions, Swiss cheese and Brie cheese. Divide the mixture evenly in the popover pan, pressing down gently on each to get them packed inches Bake until popovers are golden brown and puffed, about 20 to 25 minutes. Remove from the oven and let cool slightly before removing them from the pan and serving.