Toss lamb with 2 tbsp water, 1 tsp salt, 1/4 tsp pepper and baking soda until thoroughly combined, Set aside.
Meanwhile, place potatoes in medium saucepan; add water to just cover and 1 tbsp salt. Bring to boil over high heat. Reduce to medium-low and simmer until soft (no resistance to fork), 8-10 minutes. Drain and return to sauce pan. Return pan to low heat and cook, shaking pan regularly, until any surface moisture on potatoes has evaporated, about 1 minute. Remove from heat and mash well. Stir in melted butter. Whisk milk and egg yolk together in small bowl, stir into potatoes. Stir in chives. Season with salt & pepper to taste. Cover & set aside.
Heat oil in broiler safe 10" skillet over medium heat until shimmering. Add onion, mushrooms, 1/2 tsp salt, and 1/4 tsp pepper; cook, stirring occasionally, until veggies are starting to soften and dark bits are forming a font on bottom of skillet, 4-6 minutes. Stir in tomato paste and garlic; cook until font on bottom of skillet is dark brown, about one minute. Add Port and cook scrapping up font, until fully evaporated, about one minute. Stir in flour and cook one full minute. Add broth, Worcestershire Sauce, thyme, bay, and carrots; bring to boil, scrapping up font. Reduce to medium-low add lamb on top in 2" chucks (do not mix in), and bring to gentle simmer. Cover and cook 10-12 minutes till meat cooked through. Stir cornstarch and 2 tsp water together and stir mixture into skillet and continue simmer for 30 seconds. Remove thyme and bay leaf. Season with salt & pepper to taste.
Adjust oven rack 5" from broiler. Placed mashed potatoes in large zip-lock bag (or pastry bag) and snip on corner to make one inch opening. Pipe potatoes over skillet in an even layer making sure to cover entire surface. Smooth with back of spoon , then use tines of fork to make ridges over entire surface. Place skillet on baking sheet and place under broiler until golden brown. Let cool 10 minutes before serving. Enjoy.