Preheat oven to 350 degrees. Grease a 9x9 square baking dish with non-stick cooking spray.
Whisk milk, dijon,eggs and salt together in a medium bowl. Set aside.
Add half of the croissants to the bottom of the baking dish, cover with half of the broccoli, followed by half of the cheese. Repeat. Pour egg mixture over. Cover with foil and let egg mixture soak up the bread for 30 minutes to one hour. (If you're short on time, you can skip this step, but you may need to bake longer.)
Bake, uncovered for 45 minutes or until golden brown, puffed and cooked through.