2. Place the onions in a saucepan with cold salted water to cover. Bring to a boil. Cook the onions until just tender, about 5 minutes. Drain well, refresh under cold water and drain again. Peel the onions.
3. If using rosemary, strip the bottom 2 to 3 inches of leaves off the stems (slide them off between your thumb and forefinger).
4. Assemble the spedini. Skewer the chicken and onions on the rosemary or bamboo skewers, placing proscuitto and sage leaves between each. Use a slender bamboo skewer to make starter holes in the chicken and onions.
5. Make the basting mixture. In the bottom of a bowl, mash the garlic and salt to a smooth paste with the back of a wooden spoon. Stir in the lemon juice, oil and water.
6. Preheat the grill to high. Grill the spedini until the chicken is cooked through, about 3 to 4 minutes per side, basting with the garlic mixture.