First start the potatoes by adding them to generously salted cold water (about 2 tablespoons of salt). You can leave them refrigerated until ready to make or proceed to 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 the oil in a large oven-proof skillet over medium heat. Add the onion, lightly season with salt and saute until mostly translucent and tender, 5-7 minutes (adding a scant more oil only if needed, keeping in mind the meat will be added soon and will release some fat). Add the beef, breaking the meat up into small pieces using a fork. Season lightly with salt and pepper to taste (noting the wine, broth, and Worcestershire will add flavor and salt). Add the rosemary and thyme. Cook until the meat is mostly no longer pink but still not cooked through, about 5 minutes (it will finish cooking as you proceed).
Add the garlic, stirring until fragrant, 30 seconds or so.
Turn off the heat and add the Guinness or red wine, stirring to deglaze the pan. Return to medium heat and add the beef broth and Worcestershire. Bring the mixture to a low boil, 5 minutes.
Add the chopped vegetables. Slowly sprinkle the cornstarch, while simultaneously stirring to mix well. Allow the mixture to simmer and thicken to desired consistency, which should only be a thickened essence, not soupy and not quite a gravy. Adding more broth only if necessary. Turn off the heat and set aside. If it's too loose, simmer a little longer, sprinkle a little more cornstarch, but only if necessary.
Drain the potatoes and return them to the hot pot over very low heat (this will help to dry the potatoes and remove excess moisture). Add half of the warm milk and begin to mash by hand using a potato masher. Add about half of the melted butter and continue to mash. Add in the rest of the milk and butter, a little at a time until mashed as desired. Adjusting with more or less milk and butter as needed. Fluff with a fork.
To Assemble:
Spread the beef mixture out evenly in the skillet. Layer the mashed potatoes on top, smoothing evenly across the meat mixture. You can use the tines of a fork to make a spiraled texture to the top if you'd like.
Place in a preheated oven for 40-50 minutes, or until the potatoes begin to turn golden.
Remove from the oven and garnish with parsley. Serve hot.
Notes
The idea is to use a cup of chopped vegetables, feel free to use a combination of carrots, peas and corn, etc.**Potatoes can be added to a pot of cold and generously salted water and refrigerated (for a few hours or overnight) until ready to bring to a boil, noting that it will take a bit longer to come to a boil when it is cold, so factor that into your time. Mashed potatoes can also be made first and kept on the stove before proceeding to prepare the meat mixture.Depending on the leanness of the meat, more or less fat will be rendered. Feel free to drain off some of the fat leaving behind a tablespoon in the skillet. Consider adding scallions to the potatoes by chopping and soaking the scallions in the milk. As the milk warms the scallions will infuse the milk and add a nice addition to the mashed potatoes.