Step 1Heat the oven to 400 degrees. With a small, sharp knife, pierce each potato. Bake the potatoes for 45 minutes, or until fork tender. Remove from the oven and allow to cool slightly, then refrigerate until cold.
Step 2Peel the potatoes and cut into one-half-inch cubes. Reserve. Heat a large saute pan over medium-high heat, and add half each of the butter and oil. When the butter is melted, add half of the potatoes, distributing them so they lie comfortably in a single layer. Season with one-half teaspoon salt and one-eighth teaspoon pepper, or to taste. Cook, stirring occasionally, until the potatoes are golden-brown, 8 to 10 minutes. Add half each of the shallots and garlic and cook an additional 30 seconds. Transfer the potatoes to a plate, cover and keep warm.
Step 3Add the remaining butter and oil to the pan and brown the rest of the potatoes the same way, adding the rest of the shallots and garlic at the end.
Step 4While the potatoes are cooking, stir together the cream and mustards until blended. When the potatoes are done, stir in the cream mixture and allow it to come to a simmer. Remove the skillet from the heat, add back the reserved cooked potatoes, and stir in the sage. Adjust the seasoning if necessary, and serve immediately.