In a large pot, heat 1 tablespoon of the grapeseed oil. Add the chopped onion and fennel and cook over moderate heat, stirring occasionally, until tender and lightly browned, about 15 minutes. Add the wine and cook until evaporated. Add the pear, peppercorns, thyme sprig, bay leaf and roast squab carcasses, along with any fat in the pan. Add 8 cups of room-temperature water and bring to a boil. Simmer over moderate heat, skimming, until the stock has reduced by half, about 1 1/2 hours. Strain the stock into a large saucepan and simmer over moderate heat until it is reduced to 1 cup, about 30 minutes.