Step 1Scrub potatoes under cold running water and pat dry with paper towels. Pierce each several times with fork. Bake at 425 degrees until potatoes are fork tender, about 1 hour.
Step 2Wrap garlic in foil and bake at 350 degrees until cloves are tender, about 30 minutes. Remove peel from roasted garlic cloves.
Step 3Cut horizontal slice off top third of each potato. Scoop pulp from potatoes into bowl, leaving shells about 1/4-inch thick. Scoop pulp from tops of potatoes into bowl. Spoon potatoes and cooked garlic cloves, a portion at a time, into potato ricer and press through, into bowl. Stir in buttermilk, sage, rosemary, thyme, salt and pepper to taste.
Step 4Spoon mixture evenly into 4 shells. (At this point, potatoes can be refrigerated, covered, and baked the next day.) Brush tops with egg white. Bake at 425 degrees until tops are lightly browned, 20 to 25 minutes.