In a large pot add the potatoes and cover with water. Add 2 tsp of salt and boil until the potatoes are soft. Drain the water from the pot. Add 2 tbsp unsalted butter, 1/4 milk and 1/3 cup sour cream. Mash until smooth. Cover while filling is being made.
Preheat oven to 350F
Over medium-high heat add 1/2 tbsp of olive oil to a large pan and 1 lbs of beef, sprinkle with half of the salt and pepper. Brown meat in a large pan breaking it up with a wooden spoon into small pieces. Brown 1 pound at a time so meat browns nicely. Too much meat in the pan will cause it to boil in it's own juices and turn it an icky grey. Once the first lb is browned and minced, transfer to a dish while you work on the 2nd lb sprinkling with the remaining salt and pepper. Remove the second lb of beef to the dish with the rest of the beef.
Add another tbsp of olive oil to the pan and add in the diced onion, garlic and 2 tsp fresh thyme. Cook until the onions become translucent and soft. Add the beef back in the large pan, add in the 2 cups of frozen veggies, 3 tbsp of tomato paste, 3/4 cup stout, 2 tsp Worcestershire sauce and 1/2 cup beef broth. Mix to combine.
Let simmer for 5-7 minutes until half of the liquid has been absorbed or boiled down. The alcohol will boil off of the beer during this step.
Add the beef mixture to a 9" X 11" casserole dish or 11" round casserole dish. Spread evenly over the bottom of the dish.
Top evenly with the mashed potatoes using a spoon to create small peaks of the mashed potatoes. These peaks will brown during the baking process adding texture and flavour.
Bake for 20 minutes. You may wish to place a baking sheet on the rack below to avoid any spill-over messes. Turn to broil for the last 5 minute to brown the top of the potatoes. Watch this step carefully so you don't burn the top. It should just become golden with some darker browning on the peaks.
Remove from the oven and let rest 10 minutes before serving.
Storage: Cottage Pie can easily be made in advance and stored in the fridge for up to 5 days once baked. Reheat in the microwave or in an oven-safe dish.You can prepare, cover and store in the fridge for up to a day before baking.You can also freeze cottage pie for a few months and bake from frozen until the centre is hot. If the potatoes are browning too much and the pie isn't heated through, just cover with foil and continue baking.Substitutions:Beer can be substituted for additional beef broth.Frozen veggies swapped for fresh chopped.