Add the potatoes to a large pot and fill with water. Bring the water to a boil over high heat. Reduce heat to a simmer and cover, continuing to cook the potatoes until fork tender (but not so tender that they are falling apart).
Carefully drain the potatoes using a large colander, reserving 1 cup of water for later use.
Return the pot to the stove over medium heat. Add 1 tablespoon of butter and the diced shallots. Sauté the shallots with the butter for 3-4 minutes, or until softened. Mix in the garlic with the shallots and saute for an additional 1 minute.
Reduce heat to medium low and add the chicken stock (or reserved water) and the milk. Heat until just warm (not boiling) and carefully transfer the potatoes back to the pot. Use a large potato masher to mash the potatoes with the shallots and the milk.
Add the Greek yogurt (or sour cream), cheeses, remaining butter and salt + pepper, to taste. Continue mashing and mixing until desired consistency is reached.
Cover pot and set aside.
In a small skillet over medium heat add approximately 1/4-1/2 cup olive oil and heat until hot. Carefully add 6-8 sage leaves to the skillet and fry in the oil for 3-4 seconds before gently removing with a fork to plate lined with paper towels. Sprinkle with coarse sea salt, if desired.
Once the sage has cooled crumble over the potatoes and mix well to combine.