Begin by preparing the reindeer meat. If using frozen meat, let it thaw just enough to slice very thinly across the grain, partially frozen meat is easier to handle and slice cleanly. Set aside.
In a heavy bottomed pot or cast iron casserole, melt 1 tbsp of butter over medium to high heat. Once sizzling, add half of the meat slices, browning them quickly without crowding. Remove and repeat with remaining meat and butter.
Lower the heat to medium and add the onion slices to the same pot. Sauté gently for about 5–7 minutes until softened and lightly caramelised, scraping up any fond left by the meat.
Return all browned meat to the pot. Pour in the beer and beef stock, stirring to deglaze the base thoroughly. Add the crushed juniper berries, salt, and pepper, giving the mixture a slow stir.
Bring the pot to a simmer, then cover and let it cook gently over low heat for 1 hour. Stir occasionally to prevent sticking. The meat should become tender and absorb the aromatic broth.
If a thicker consistency is preferred, mix 1 tbsp of flour with a splash of cold water to form a slurry. Stir it into the pot during the last 10 minutes of cooking, allowing the sauce to gently thicken.
Meanwhile, prepare the mashed potatoes. For best results, use starchy potatoes like Maris Piper. Boil until soft, mash with warm milk and butter, and season with salt to taste.
Taste the stew and adjust the seasoning as needed. Add a touch more salt or black pepper if the beer's bitterness is too pronounced. Let it rest covered for 5 minutes before serving.
To serve, spoon generous portions of sautéed reindeer over a bed of hot mashed potatoes. Add a dollop of lingonberry jam on the side, the tartness provides the perfect contrast to the savoury richness. For a rustic touch, garnish with a few juniper sprigs or finely chopped parsley.