Preheat your broiler. After thoroughly washing your vegetables, ensure they are completely dry. Dice the potatoes into half-inch pieces. For the carrot, trim, peel, and halve it lengthwise before slicing into quarter-inch thick half-moons. The celery should be finely chopped. For the onion, halve, peel, and finely chop it. Remove the leaves from the thyme stems and roughly chop, do the same for the parsley.

Place potatoes in a medium pot and cover with salted water by 2 inches. Boil until tender, about 15-20 minutes. After draining, mash potatoes with sour cream, butter, and one teaspoon of chopped thyme until smooth. Add water as needed to achieve a creamy texture. Season well with salt and pepper. Keep the mashed potatoes covered and off heat until needed.

While potatoes cook, heat a medium-sized, oven-safe pan with some oil on medium-high. Add the carrot, season with salt and pepper, and sauté for 2-3 minutes until it begins to soften. Then, add celery and onion with more oil, seasoning again with salt and pepper, and sauté for 5-7 minutes until the veggies are somewhat tender. If they brown too fast, add a bit of water. Lastly, stir in the garlic powder and 2 teaspoons of chopped thyme, cooking until the fragrance fills the kitchen, about 30 seconds.

Introduce the beef* to the pan containing the vegetables and season with salt and pepper. Stirring frequently, allow the meat to break up into pieces and cook until it turns brown and is thoroughly cooked, which should take about 4-6 minutes. Incorporate the tomato paste and flour into the mix, stirring constantly, until they are fully integrated, which should take approximately a minute.

Gradually pour ½ cup of water into the pan of beef, blending in the stock concentrates. Bring it to a boil, and let it thicken for about 1-2 minutes. Switch off the heat, stir in half of the parsley, and season as desired. If your pan isn't oven-safe, transfer the mixture to a baking dish now.

Cover the beef filling with a uniform layer of mashed potatoes, leaving a small space around the pan's edge. Scatter cheddar evenly over the top. Broil until it turns brown, which should take around 3-4 minutes. Be vigilant to prevent it from burning. Garnish with the remaining parsley and serve straight from the pan.