Chop up potato and add to a small saucepan and cover with water. Bring to a boil and cook until the potato can be pierced with a fork.
Once the potato is about done, add frozen vegetables and cook a few more minutes until they begin to soften. Remove from heat, drain, and pour into a large mixing bowl.
In a large saucepan, melt butter. Add chopped onion and cook until softened, 2-3 minutes.
Stir in flour, salt, and pepper and cook until bubbly.
Add stock and milk and bring to a boil, stirring constantly. Cook and stir until slightly thickened, 1-2 minutes. Remove from heat and pour over vegetables.
Add cooked meat to the bowl and stir until combined. Pour into ramekins or pie plate.
Top with pie or biscuit crust of choice.
Bake at 425 degrees F for 40-45 minutes (pie plate) or 15-20 minutes (ramekins) or until crust is slightly brown and filling is bubbly.
Allow to cool for several minutes before serving.
Store leftovers in an airtight container in the fridge for up to three days.