Preheat the oven to 375 degrees F.

Make mashed potatoes: Add potatoes to a saucepan with 3 inches of water. Cover and bring to a boil. Cook until fork tender, about 12-15 minutes.

Drain the water and leave the potatoes in the pot. Cover with the lid and allow to steam for a few minutes.

Mash the potatoes lightly with a potato masher (avoid over-mixing the potatoes). Add butter, sour cream, milk, parmesan cheese and salt and pepper and stir just until smooth. Set aside.

Make the meat gravy: In a large, oven-safe* 12 inch skillet with sides, add the ground beef and ground sausage (or ground lamb, to make shepherd's pie). Season with salt and pepper.

Cook over medium heat until browned, breaking the meat into small pieces with a wooden spoon as it cooks. Drain some of the grease (reserving some in the pan) and remove meat to a plate.

Add the onion, celery and carrots to the pan and sauté for about 5-8 minutes, until tender. Add garlic and flour and toss to combine. Cook for 1 minute.

Add the beef broth, bouillon cube, tomato paste, Worcestershire sauce, herbs, and bay leaves and stir to combine.

Bring to a simmer and cook for 5 minutes. Taste and add extra seasonings, if needed. Remove bay leaves and stir in frozen peas, corn, and cooked meat.

Assemble: Spoon the mashed potatoes over the meat mixture to completely cover the top. Sprinkle with shredded cheddar cheese.

Bake for about 30 minutes, until golden and bubbling. Allow to cool for 15 minutes before serving, to allow the gravy sauce in the meat mixture to cool and thicken.

Store leftovers in the refrigerator for 3-5 days.