Place a pan on the stovetop and add 3 tbsp of olive oil and allow the oil to heat up.
Once heated, add the chopped onions and saute for about 3 minutes until the onion is slightly wilted.
Next, add 3 minced garlic cloves, and also saute until it is fragrant.
Now, add the remaining vegetables: diced celery sticks and carrots; cook until the carrots are tender. This should take about 3 minutes.
Go in with the ground lamb or mutton and use a spatula to break it apart into tiny pieces. You need to do this quickly because once heated, ground lamb or meat can form balls that may be hard to break apart. Cook for 6 minutes until the lamb is no longer pink in color.
Season the lamb with dried rosemary, chopped fresh thyme, 1/4 tsp of black pepper, and a pinch of salt.
Also, add frozen peas and lamb stock. You can substitute the lamb stock with a mixture of bouillon cubes dissolved in water; you can use a gravy for this also. Stir everything to combine and then cover and cook for 12 minutes until the lamb is tender.
Add 1 tbsp of Worcestershire sauce and 1/4 tsp of red chili pepper for more heat. This is optional if you don't want pepper. Stir to combine.
Next, make a slurry of cornstarch by adding 3 tbsp of water to 2 tbsp of cornflour/cornflour; pour the slurry over the meat filling in the pan and stir immediately to avoid the formation of lumps by the cornflour mixture when heated. The cornstarch helps to thicken the filling of the Shepherd pie.
Cover the filling and cook for 2 minutes until it thickens up and then turn off the heat.
Pour the filling into a casserole dish or your baking dish.
Use a spatula or spoon to level the top of the filling to become flat.
Scoop in the mashed potatoes over the filling so that the mashed potatoes completely cover the pie filling. Also, use a spoon to spread the mashed potatoes and then flatten the top.
You can use a fork to draw over the top of the mashed potatoes so that it forms stripes. This helps to give a crispy crust to the shepherd's pie.
Preheat your oven to a temperature of 400 degrees Fahrenheit for 20 minutes or when you notice the golden brown surface of your Shepherd's pie.
You can bring it out of the oven and your easy homemade Shepherd's pie is ready!