Prep the potatoes. Peel and cut potatoes into equal chunks. Add to a pot of cold salted water. Bring to a boil and cook until fork-tender, about 15–18 minutes.

Make the filling base. While potatoes cook, heat 1 tablespoon oil in a large skillet over medium heat. Add the chicken, season with salt and pepper, and cook until lightly browned and just cooked through, 5–7 minutes. Remove to a plate.

Sauté the vegetables. In the same pan, add remaining oil and the onion, carrots, and celery. Cook until softened, about 6–8 minutes. Stir in garlic and cook 30 seconds.

Build the gravy. Sprinkle flour over the vegetables and stir for 1 minute. Add tomato paste and mix well. Gradually pour in chicken broth, whisking to avoid lumps. Add thyme, bay leaf, and Worcestershire. Simmer 3–5 minutes until slightly thickened.

Finish the filling. Return the chicken and any juices to the pan. Stir in peas and dried parsley. Taste and adjust seasoning with salt and pepper. Remove bay leaf. The mixture should be saucy but not soupy.

Make the mashed potatoes. Drain potatoes well. Mash with butter and 1/2 cup milk, adding more milk as needed for a smooth, spreadable texture. Season with salt and pepper. For extra richness, add a handful of cheddar.

Assemble. Spoon the chicken filling into a lightly greased 2-quart baking dish. Dollop mashed potatoes over the top and spread to the edges to seal. Rough up the surface with a fork for extra crispy bits. Sprinkle with paprika or cheese if using.

Bake. Place in a 400°F (200°C) oven for 20–25 minutes, until the top is lightly golden and the filling is bubbling around the edges. For deeper color, broil 1–2 minutes at the end.

Rest and serve. Let the pie rest 10 minutes to set. Garnish with fresh parsley and serve warm.