Place the sweet potatoes in a large pot, cover with water and bring to a boil. Salt the water and cook the potatoes until tender, about 18 minutes; drain. Return the potatoes to the hot pot and mash with the milk, 1 tablespoon butter, the egg and paprika; season with salt and pepper.
Preheat the broiler and position a rack in the lower third of the oven. Place a heavy pot or large dutch oven on medium-high heat. Add the EVOO, 1 1/2 turns of the pan, and the mushrooms and cook until darkened, 6 to 7 minutes. Add the sirloin and cook until browned, 5 minutes. Add the onion, garlic and sage; season with salt and pepper. Cook until the onion is softened, about 5 minutes. Stir in the peas.
While the onion cooks, heat a small saucepan or skillet over medium-high heat. Add the remaining 2 tablespoons butter to melt, then whisk in the flour for 1 minute. Add the beer, if using, and cook until reduced, a minute or two. Add the beef stock and worcestershire; season with salt and pepper. Cook until thickened slightly, then whisk in the mustard. Stir the gravy into the meat..
Pour the meat and gravy into a casserole and top with the sweet potatoes. Broil until the potatoes are set and lightly golden-brown at the edges. Top with the parsley.