Preheat your oven to 400º.
Bring a large pot of water to a boil. Add the cut Yukon Gold potatoes and continue to boil for 20-25 minutes until the potatoes are easily pierced with a fork.
While the potatoes are boiling, add the mushrooms and 1/2 cup of water to a large saucepan or stockpot. Simmer until the water has cooked off. About 6-8 minutes. When the water has cooked off, add 1 tbsp olive oil and briefly sauce the mushrooms until they begin to brown. Set aside.
Add the remaining 2 tbsps of olive oil to the large sauce pan or stockpot. Heat until shimmering then add the onions and salt and stir to coat. Reduce the heat to medium-low and slow cook the onions until they are golden brown. About 10-12 minutes.
Add the garlic, mushrooms, rosemary, parsley, thyme and tomato paste and stir to coat. Increase the heat to medium-high and sauté until fragrant and the tomato paste has turned a dark red.
Add the carrots, celery, parsnips, Worcestershire sauce and lentils and stir to coat. Add the water and simmer for 10-15 minutes until the water cooked down.
While the Shepard’s Pie filling is simmering, drain the potatoes and add them back to the large pot along with the vegan butter, coconut milk and salt.
Using a hand mixer, combine all of the ingredients until creamy.
Spread the filling into a large casserole dish. The dish should be about 2/3rds full. Then spoon the mashed potatoes over the filling and spread to cover. The potatoes should fill the remaining 1/3rd of the dish.
Bake, covered, for 30 minutes. Continue to bake, uncovered, for an additional 20 minutes.
Allow the casserole to cool slightly, then serve with fresh chives and salt & pepper, to taste.