Heat oven to 350° F.
Place the cubed potatoes in a pot and cover with cold water and 1 teaspoon salt. Bring to a boil and reduce to a simmer.
In a large saucepan, bring the beef broth, pearl onions, garlic, thyme leaves, and rosemary to a simmer.
Whisk together the flour and water until smooth. Then whisk the flour slurry into the simmering broth.
Add the mushrooms.
Add the wine and Worcestershire sauce.
Add the roast beef or lamb, ½ teaspoon salt, ½ teaspoon pepper, and peas. Bring back to a simmer and cook for 5 minutes. Then turn off the heat and set aside.
Check the potatoes with a fork. If you can pierce the potatoes easily, then they are done. Drain them and return to the pot. Add 4 Tablespoons of the butter and mash the potatoes.
Mix in the heated milk and ½ teaspoon of salt.
Add the egg yolk and remaining ½ teaspoon of pepper. Mix well.
Pour the stew mixture into a 3 quart casserole dish.
Spread the mashed potatoes over the stew mixture, covering completely, edge to edge.
Melt the remaining butter and pour over the top of the mashed potatoes.
Bake for 20 to 30 minutes, or until the potatoes start to brown and the stew is bubbling a little.