Position a rack in the middle of the oven and preheat to 450F. Line a baking sheet with parchment paper. Place the meat in a large bowl and season liberally with salt and pepper. In a small bowl, soak the bread in the milk then crumble the bread into the meat.
Mix the scallion whites, parsley, mint, garlic, egg, lemon zest and a healthy drizzle of EVOO into the meat. Separate the mixture into 4 equal portions. Using your hands, form 4 mini loaves about 1 1/2-inches thick. Place the loaves on the prepared baking sheet and roast in the oven until cooked through, 15-18 minutes.
Meanwhile, in a pot, combine the potatoes and enough cold water to cover; bring to boil. Salt the water and cook potatoes until tender, 15-20 minutes. Drain the potatoes and return to the hot pot. Add the half-and-half, scallion greens, and 2 tablespoons butter; mash to your desired consistency. Season with salt and pepper then fold in the peas. Cover the pan to keep warm.
In a small skillet, melt the remaining 2 tablespoons of butter. Whisk the flour for 1 minutes then whisk in the beef stock (or consomm) and Worcestershire; season with pepper. Cook to thicken the gravy for a couple of minutes, then whisk in the mustard.
Serve meatloaf with the mashed potatoes. Top with gravy.