On medium heat, Brown the meat in a large pot (Dutch Oven) Add the Mushrooms, onions, garlic, celery and carrots. Let these cook down for about 5 minutes stirring every so often.
Add the flour- stirring to distribute it. Add in the broth and stir until the flour is dissolved. Add in the tomato paste, parsley, thyme, Worcestershire sauce and salt and pepper.
Mashed Potatoes
Wash and Peel your potatoes then cut them lengthwise and add them to a pot.
Turn the heat to high and bring the potatoes to a boil once they are boiling turn down the heat and simmer the potatoes for 20-25 mins until tender and can be pierced easily with a fork.
Once cooked, strain the potatoes and add back to the pot on the stove and cook off and remaining moisture.
Add milk, butter and seasonings.
Use a potato masher either manual or electric and mash the potatoes. Don't over do it though, when you work them too long they turn to glue!
Assemble!
in a Deep 9x13" dish or two 2.5qt dishes spray the dish with non stick cooking spray. Add the meat and gravy to the bottom. Top with the frozen vegetables and top those with some salt and pepper. Dollop the mashed potatoes on top of the vegetables and then spread them out evenly. You can then bake the Shepherd's Pie at 400°F for 30-45 Minutes. If you are adding grated cheddar cheese on top of the potatoes, do so in the last 10 minutes of cooking.
Notes
You can mix your frozen veggies into your meat and gravy combo or have them as their own layer. I always switch between the two its whatever I feel like at the time of making it. There isn't too much of a difference in the final product when you alternate methods, so do whatever you feel like. :)