Break up the ground beef in a medium mixing bowl. In a small cup combine the baking soda with 2 tablespoons water, ½ teaspoon salt and ¼ teaspoon pepper. Stir to dissolve. Pour the baking soda mixture over the ground beef and toss it all to combine. Set the meat aside for 20 minutes at room temperature.

In a large skillet (at least 12 inches wide) add the butter and olive oil. Heat over medium until butter is melted and just starting to foam. Add the onion, leek, celery, carrots, ½ teaspoon salt, ¼ teaspoon pepper and paprika, if using. Cook, stirring occasionally, until the celery and carrots start to soften, about 7 minutes.

Add the minced garlic and tomato paste and stir constantly until the mixture darkens and the garlic is fragrant, about 2 minutes.

Sprinkle the flour over the vegetables and stir to combine. Cook, stirring constantly for about 2 minutes, scraping up the browned bits on the bottom of the skillet.

Add the beef broth, Worcestershire sauce, parsley, rosemary and thyme. Scrape the bottom of the pan to release any browned bits and bring to a boil, still over medium heat.

Reduce the heat to medium-low and spread out the vegetables evenly. Add the beef in small portions on top of the vegetables. Bring the mixture to a simmer, cover and cook until the beef is cooked through, about 12-15 minutes, stirring regularly to break up any large clumps.

Whisk together the cornstarch with 1 tablespoon water and add it to the beef. Cook for 1 minute or until slightly thickened. Stir in the peas and remove the skillet from the heat. Check the seasoning and add salt and pepper as needed. The filling can be made ahead and stored in the fridge till needed. Return to room temperature before assembling.

Pre-heat the oven to 425F.

Using a fork, pierce steam holes evenly over each potato. Rub with a little olive oil and sprinkle with salt.

Place the potatoes directly onto a rack set in the centre of the oven. Bake for 50-60 minutes, depending on the size of your potatoes. Once a knife slides in right to the middle, they are baked through.

Remove and cool till you can handle them. Slice off the top section, exposed enough of the interior that you can get a spoon in to scoop it all out.

Scoop out the filling, but leave a wall of flesh around the exterior so that the potato will maintain its structure once filled with shepherd's pie filling and placed in the oven to continue baking.

Place all the scooped out filling into a bowl. Add the butter, milk, salt, pepper, nutmeg and cheddar cheese. Blend with a fork till rustically smooth. Taste and re-season as desired.

Place all the hollowed out potatoes onto a baking sheet. Spoon in the filling, packing it in without pressing so hard that the potato breaks.

Divide the mashed potato mixture between the potatoes. Fluff the tops up lightly with a fork, creating a pattern of sorts.

Sprinkle with more cheese and parmesan cheese, and chilli flakes if desired.

Bake until the cheeses have melted and are starting to bubble, about 20-25 minutes.

Sprinkle with chopped parsley or chives, and more chilli flakes if desired.

Serve with a side salad or roasted or steamed broccoli.