1. Coat a 4.5-quart slow cooker with cooking spray.
2. Heat a large skillet over medium-high heat. Add the sausage, onion and oregano; break the meat into small pieces and cook until the sausage is browned and no longer pink, about 10 minutes.
3. Chop steamed spinach and kale, mix well in a small bowl; set aside.
4. Add spinach and kale to the sausage mixture. Transfer to the slow cooker. Add potatoes, 1 cup of the Monterey Jack cheese, and Parmesan cheese and stir to combine.
5. Place the eggs, milk, mustard in a large bowl and whisk until smooth. Season with salt and pepper. Pour the egg mixture into the slow cooker and stir until the egg is evenly distributed. Smooth the filling into an even layer.
6. Cover and cook on the low setting until the eggs are set, about 4 to 6 hours.
7. Turn off the slow cooker and sprinkle the remaining 1/2 cup Monterey Jack cheese over the top of the casserole.
8. Cover and let sit until the cheese melts completely before serving with pepper flakes, about 5 minutes.