Preheat oven to 375 degrees F. Line a 15x10x1-inch baking pan with parchment paper. Rub potatoes with oil and place in prepared pan. Bake for 50 to 60 minutes or until tender when pierced with a knife.
Meanwhile, in a small saucepan combine 1 cup water, mustard seeds, 1 tablespoon vinegar, and 1/2 teaspoon salt. Bring to boiling. Reduce heat to medium. Simmer, uncovered, for 10 minutes. Remove from heat and pour through a fine mesh sieve into a bowl, reserving seeds.
In a large skillet melt butter over medium heat. Add sage leaves (butter may spatter); cook about 3 minutes or until curled and crisp, turning the leaves occasionally. Transfer leaves to a paper towels to drain and sprinkle with 1/8 teaspoon salt. Continue to cook butter in skillet 2 to 3 minutes or until lightly browned. Immediately transfer butter to a medium bowl.
Whisk reserved mustard cooking liquid, remaining 1 tablespoon of vinegar, and remaining 1/2 teaspoon salt into browned butter. Whisk in reserved mustard seeds.
Using the bottom of a drinking glass or measuring cup, gently smash the hot potatoes into 1/2-inch thick disks. Transfer to a serving plate and drizzle with mustard vinaigrette and top with sage leaves. Sprinkle with sea salt flakes.