Line sides of 5-quart slow cooker with foil that has been folded into thirds; spray with cooking spray.
In medium bowl, beat eggs, milk, pepper flakes, salt and pepper with whisk.
In small bowl, stir together Monterey Jack cheese and ¼ cup of the Cheddar cheese.
In slow cooker, layer half of each of the potatoes, sausage and roasted peppers; top with 3 tablespoons of the onions and half of the cheese mixture.
Repeat layers.
Pour egg mixture over top.
Cover; cook on Low heat setting 4 hours to 4 hours 30 minutes or until temperature reaches 160°F in center and egg mixture is set.
Turn slow cooker off.
Sprinkle remaining ¾ cup Cheddar cheese and remaining 2 tablespoons onions over casserole.
Cover; let stand 10 minutes or until cheese is melted.
Remove foil before serving by loosening edges with table knife and pulling out.