Preheat oven to 400 degrees. Prep a square pan by lining it with aluminum foil. Set aside. Wash sweet potatoes and dry them well.
Grab one potato and two wooden spoons. Place a spoon on either side of the potato in the same direction. About 1/2 inch from the end of the sweet potato, slice down with a sharp knife in one forceful motion until you hit the spoons. Move the knife over about 1/4 inch and repeat. Continue with the rest of the potato. Repeat with 3 other potatoes.
Place 4 potatoes into prepared pan. Drizzle potatoes with a little olive oil and sprinkle generously with salt, making sure to season in between the slices. Bake in preheated oven for 45 minutes.
While potatoes are baking, prepare the sage & browned butter sauce. Slice butter and place into small saucepan and heat on medium. When butter begins to bubble, add chopped sage leaves and continue to cook for 5 minutes. Do not stir. Butter should begin foaming and the butter should be turning brown. Once it's deep golden brown and most of the foam is gone, turn off heat. Sage leaves should be crispy but not burnt. Transfer sauce to heat-proof bowl to keep the butter from continuing to cook. Squeeze a little of the lemon into sauce and stir.
Once potatoes have been baking for 45 minutes, remove from oven and check if they're ready. They should be fork-tender. If not, bake for 10 more minutes or until they are.
Finish off potatoes by spooning browned butter and sage leaves into the sliced pieces of each potato (if butter solidified a bit, simply microwave for 5-10 seconds to melt again). Make sure to get in between the slices! Serve immediately.