Preheat oven to 375 °F.

Heat a large, cast-iron skillet or Dutch oven over medium-high heat with olive oil, 2 turns of the pan. Add the mushrooms and brown; add the meat and brown and crumble it. Add the carrots, parsnips, celery, onions and garlic, and season with salt, pepper and herbs. Partially cover the pot and cook 7-8 minutes to soften the vegetables. Sprinkle 3 tablespoons of flour over the meat and vegetables, and stir a minute; add Worcestershire and stock. Cook to thicken a bit then transfer to a casserole dish.

Bring a pot of water to a boil, salt it and par-cook potatoes 5 minutes. Drain well and pat them dry.

In a saucepot over medium heat, melt butter and whisk in remaining 3 tablespoons flour. Add milk and cream, and thicken. Season with salt, pepper and nutmeg to taste. Stir in cheese to melt.   

Shingle half the potatoes over the meat and vegetables in the casserole dish then top with half the sauce. Shingle remaining potatoes and top with sauce. Sprinkle with cheddar cheese and bake until brown, bubbly and golden.