First start the potatoes by adding them to generously salted cold water. Bring to a boil until they are fork tender, about 20 minutes.*
While the potatoes are cooking, preheat the oven to 350 F. (177 C.).
Heat oil in a large skillet over medium heat. Add the onions, lightly salt and saute until mostly translucent and tender, 5 minutes. Add the beef, breaking the meat up into small pieces using a fork. Cook until semi-through, 5-7 minutes, or until it is mostly no longer pink.
Add the garlic, stirring until fragrant, 30 seconds or so.
Turn off the heat and add the red wine, stirring to deglaze the pan. Return to medium heat and add Add beef broth and Worcestershire. Season with salt and pepper, stir to combine. Allow the mixture to come to a simmer.
Add the vegetables. Slowly incorporate the cornstarch, while simultaneously stirring to mix well. Allow the mixture to simmer and thicken to desired consistency, which should be a thickened essence, not soupy. Turn off the heat and set aside. If it's too loose, simmer a little longer, adding more cornstarch only if necessary.
To prepare the potatoes:
In the meantime, as the potatoes have just about boiled until tender; cover chopped scallions in a saucepan with cold milk and bring slowly to a boil, simmering for 3-4 minutes. Leave in the milk to infuse a few minutes longer.
Drain the potatoes, add the scallions and milk to the potatoes. Begin to mash using a potato masher. Add a stick of melted butter, a little at a time and continue to mash. Fluff with a fork. Season with salt and pepper.
To Assemble:
Spread the beef mixture out evenly in the skillet. Layer the mashed potatoes on top, smoothing out the potatoes as evenly across the beef as possible. You can use the tines of a fork to make a spiraled texture to the top.
Place in a preheated oven for 40-50 minutes, or until the potatoes begin to turn golden.
Remove from oven and top with a knob of butter. Garnish with parsley. Serve hot.
Notes
*Potatoes can be made before starting the beef mixture, sitting at room temperature until ready to use. Preparation can begin the night before, if convenient, by peeling and cubing the potatoes and leaving them in the refrigerator in a large pot covered in generously salted cold water. Place the pot on the stove when ready to cook, noting that it will take a bit longer to come to a boil when it is cold, so factor that into your time.