How to Make It
Preheat a large pan on the stovetop over medium-high heat. Crumble in the sausage and cook for 7 to 8 minutes, or until crispy and fully cooked through. Drain the sausage on paper towels.
Generously grease the inside of the slow cooker with cooking spray (or use a slow cooker liner). Spread tater tots in the slow cooker, followed by the sausage.
Whisk together eggs, milk, mustard, salt and pepper in a large bowl. Add cheese and onions. Mix well. Pour egg mixture over sausage and tater tots.
Cover and cook on low at least 4 hours and up to 5 hours, or until the eggs are fully set. I recommend keeping an eye on the casserole during the last hour or so of cooking—especially if your slow cooker runs hot—to make sure the edges don't burn. Serve straight from the slow-cooker.