While the sweet potatoes are baking, in a small sauce pan add the duck fat. Warm up on medium low.
Add the sage leaves to the heated oil and fry for about 60-90 seconds. Remove and set aside on paper towel.
Add the chopped garlic to the oil and turn off the heat. Immediately add the olive oil to cool the duck fat slightly. Set aside.
Once the potatoes are baked and tender all the way through, peel them and discard the peels. Add the flesh to a bowl and mash with a wooden spoon. Add 1-2 Tbsp of the duck fat/oil mix, along with salt and pepper to taste.
Spread the sweet potato mash onto a serving platter, topped with some of the duck fat, garlic, and the sage leaves. Serve warm.