Preheat the oven to 400 F. Get a 9x13 casserole pan and foil to cover it.
Arrange your sweet potatoes in the pan, then take each one out, peel it, and do the cuts as follows. Shave off a thin slice from the side that you want to rest on the pan. Place that side down on the cutting board and place two chopsticks on either side of the sweet potato, to use as a knife-stop. Carefully grip the chopsticks and the sweet potato and use your knife to slice down, but not through, at 1/4-inch intervals. Place the potato back in the pan and continue until all are done.
Mix the onions with a tablespoon of olive oil and the rosemary, and a pinch of the salt. Sprinkle between the potatoes. In a small pot, warm the remaining olive oil, sage, salt and pepper. Use a pastry brush to baste the potatoes, opening the slices as much as possible without breaking. You can pick each one up and bend it back slightly to open, then paint into the slices. Reserve a couple of tablespoons of the seasoned oil.
When all the sweet potatoes are seasoned and in the pan, cover with foil and bake for about 45 minutes. The sweet potatoes should be tender when pierced with a knife, in the center of a thick slice.
Uncover, baste again with the reserved oil, and turn on the broiler. Slide the pan under the broiler, watching closely, and take out to baste again every minute or so. It won't take more than 3 or 4 minutes to brown the tops.
Garnish with rosemary and sage sprigs. Serve hot.