• Set the oven and prepare the pan. Preheat the oven to 400°F, adjust a rack to the center of the oven, and line an approximately 8 x 8-inch baking pan with parchment paper.
• Make the rosemary syrup. In a small saucepan combine the water, sugar, grapefruit juice, rosemary and vanilla. Bring to a boil and then turn off the heat. Stir to ensure all of the sugar has dissolved. Set aside.

Prepare the Pears

• Peel the pears and then cut them in half, lengthwise. Use a melon baller to scoop out the seeds from each half, and then use a paring knife to cut out any remaining stem and skin.

  Coat the bottom of the parchment-lined pan with a thin layer of the syrup, and place the pear halves in, round sides up, as you prepare them, spooning some of the syrup on top of each one. (This is to prevent them from turning brown.)
• Now, one at a time, remove the pear halves from the pan and use a slicing knife to cut as many slits as you can, and as thinly as you can, horizontally, into the round side of the pear half. Don't cut all the way through the pear! Your knife should stop about about 1-inch from the bottom.

  Return the sliced halves to the pan as you go, round/sliced sides up.

  Then pour the remaining rosemary syrup over them. The pears should be well coated. Use the tip of a knife to open each slit slightly to be sure the syrup goes inside. There will be excess syrup in the bottom of the pan that will go about ¼ to ½ way up the sides of the pears. This is perfect, as we'll be basting with that during the roasting process.
• Bake and baste. Place the pears in the preheated 400°F oven and set the timer for 15 minutes. Use a small ladle or a soup spoon, to baste the pears, again getting the syrup into the slits. (The slits will become more open as they roast, and it will therefore become easier to do this.)

  Continue to set the timer and baste every 15 minutes, until you've reached 1 hour. Then roast for another 30 minutes or until the pears are very tender), basting generously at that time.

  If they are not golden on top, place them under the broiler for about 30 seconds. (Watch them carefully so they don't burn.)
• Serve. Gently remove the pears from the pan with a flat-bottomed spatula, and place them serving plates. Drizzle each one with some of the remaining syrup.