Shepherd's Pie

Preheat the oven to 425 F.

Heat olive oil in a large skillet over medium to medium-high heat. Add pearl onions, toss, cover, and steam until they begin to soften, 5 to 7 minutes. Add the garlic and celery and stir. Add mushrooms and cook until they give off liquid and start to brown, approximately 5 minutes. Add 1 tablespoon of flour, stir, and cook 1 to 2 minutes. Add meat, break it up, season with salt and pepper, and cook until browned. Add stock, tomato paste, thyme, and pepper flakes and bring to a boil. Lower heat and simmer until gravy thickens and is absorbed by meat. Stir in 1/2 cup parsley and remaining flour.

Pour filling in a deep pie dish and let cool.

Spread the mashed potatoes on top of the meat. Sprinkle potatoes with Parmesan and Cheddar cheeses. Bake pie for about 15 to 20 minutes.

Remove from oven and sprinkle with remaining chopped parsley. Let stand 5 minutes before serving.

Hints and Clues

Allow all the liquid to be absorbed by the meat and vegetables so it doesn't spill out later.
If you cannot find pearl onions, just chop and add the onions you have at hand.
You can use leftover cooked hamburger meat rather than raw, or make the meat a day ahead and reheat it before cooking.
Top the pie with the Parmesan and Cheddar cheeses to help with the browning process. As the cheese bakes, it makes a nice crust on top of the potatoes.