Peel and cube the potatoes and place in a large pot of cold water. Swirl the pan a bit to encourage the potato cubes to give up their starch, and drain. Repeat the process.
Fill the pot with cold water again and add a large pinch of salt. Set aside until ready to cook.
Bring the pot to a boil, reduce heat and simmer until the potatoes are tender but not falling apart, about 15-18 minutes, depending on the size of your chunks.
While the potatoes are simmering, make the fried shallots. Place oil in a medium skillet and heat over medium high heat. Add the shallots and fry, turning often until golden brown. Remove from skillet and drain on paper towels. Pour off any remaining oil and wipe down the skillet with a paper towel.
Add 4 tablespoons of butter to the skillet and heat over medium high heat. When melted and simmering, add the sage leaves and fry until crisp - just a minute or two per side. Remove from pan. Add the sliced garlic and fry until golden brown. Pour the butter and garlic into a small bowl and set aside.
When the potatoes are cooked, drain thoroughly. Add 1 cup of the buttermilk and the remaining butter and mash well. If it still looks too thick, add more buttermilk until you get the consistency you want.
Add salt and pepper to taste.
Turn the potatoes into a bowl. Drizzle with the garlic/brown butter and top with the shallots and sage. Serve immediately.