Peel and chunk potatoes. Add to salted water, heat over high heat until boiling. Boil until fork tender. About 15 minutes.
Drain potatoes and return to pot.
Use an electric hand mixer to mash potatoes.
Once potatoes are well mashed, add butter and milk. Mix well.
Set the mashed potatoes aside until you are ready to use them.
Filling
Brown the ground venison over medium heat. Venison tends to be very lean, so you may need to add a little oil to the skillet to keep it from sticking to the pan.
Add chopped onions and cook until soft. About 3 minutes.
Add 2 tablespoons of butter to the skillet to melt.
Sprinkle flour over the butter, meat and onions. Stir until it is absorbed into the butter and meat.
Slowly add the beef stock. Let each addition be absorbed by the flour. Be careful to make sure that the mixture is smooth with each addition. You cannot get the lumps out later.
Once all the stock has been added, allow the gravy to simmer and thicken.
Stir in the mixed vegetables and remove from heat.
Assembling the Shepherd's Pie
Pour the meat mixture into a casserole dish.
Top with mashed potatoes.
Bake 30-45 until the gravy begins to bubble and the mashed potatoes are golden brown.