Spray a 5 or 6 quart slow cooker with non stick cooking spray.
Crumble and brown breakfast sausage in a medium skillet until cooked through.
Whisk eggs in a medium bowl and add them to the slow cooker.
Add heavy cream, pimientos, ground mustard, onion, and half of cheese to slow cooker. Whisk to combine.
Add frozen tater tots and browned sausage to slow cooker. Stir to combine and evenly coat.
Cover and cook on high for 2 1/2 to 3 hours, rotating insert halfway through. Check to make sure eggs are set.
Sprinkle remaining cheese on top, cover with lid until cheese has melted.
Sprinkle with sliced green onions or chives.