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 filling: 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 saute for about 5-8 minutes, until tender. Add garlic and flour and toss to combine. Cook for 1 minute.
Add the beef broth, 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 the pie: 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.
If you don’t have a skillet that’s large enough, or oven-safe, you can add the meat mixture to a 9x13 inch pan and then top it with the mashed potatoes. I just like to avoid the extra dish!
To make ahead: Cottage pie can be made up to 3 days in advance. For best results, store the mashed potatoes and filling separately in the fridge until ready to assemble the cottage pie. Before topping the pie with the mashed potatoes you may want to add a little more milk to them (and season them if needed) to soften them.To Store: Store leftover cottage pie covered in the refrigerator for 3-5 days. Adjust storage times based on how fresh your ingredients were to begin with and take care to remember that leftover meat should not be stored for more than 5 days after it was originally cooked.To Freeze: To freeze cottage pie, bake it as directed in a freezer-safe aluminum pan and allow it to cool completely. Double wrap it in plastic wrap and tinfoil and store it in the freezer for up to 2 months. To reheat, place the frozen pie in the oven at 350ºF for 1 1/2 hours.