In a heavy-bottomed dutch oven, heat the oil until it begins to shimmer. Add the lamb and brown over high heat. Season with salt and pepper and stir occasionally until browned, approximately 8 minutes. Remove the lamb to a paper towel-lined plate.

Add the butter to the pan to melt. Stir in onion, turnips, carrots, celery, garlic, thyme and water. Cover the pan and cook over medium-low heat, stirring occasionally, until the vegetables are tender, approximately 15 minutes.

Add the lamb back to the pan, along with the tomato paste. Stir and cook for 1 minute. Sprinkle the flour over the mixture and cook another minute. Add in the stock and bring the mixture to a boil over medium-high heat. Return the heat to low and simmer, stirring constantly, until the sauce is thick, approximately 10 minutes.

Add the lamb to eight 1 1/2 cup ramekins. Allow to cool.

Preheat oven to 400 degrees.

In a large saucepan over medium-high heat, cover the potatoes with water. Add garlic and bring to a boil. Cook until the potatoes are tender, approxiamtely 12 minutes. Drain the potatoes and pass through a ricer into a large bowl (or if you don't have a ricer, simply mash by hand). Stir in milk, heavy cream, butter, and rosemary. If the potatoes are not smooth and creamy, add more heavy cream as needed. Season with salt and pepper.

Spread the mashed potatoes on top of the lamb in each ramekin (or if you want to get fancy, pipe them!). Bake for 20 minutes, until the filling is bubbling.

Turn the broiler on high and broil in the upper third of the oven for 2 minutes, until the potatoes are browned. Allow to rest at least 10 minutes before serving.