In a medium saucepan, bring the chicken broth and 2 cups water to a boil over medium-high heat. Gradually stir in the grits and salt. Cover the pan, lower the heat and simmer, stirring occasionally, until the grits are thick, about 7 minutes. Pour the grits into an ungreased 9-by-13-inch baking dish and let cool slightly. Cover and freeze until set, about 30 minutes.
Meanwhile, in a food processor, combine the cheese, mayonnaise, onion and half the pimientos until smooth. Stir in the remaining pimientos and season to taste with the cayenne.
Invert the grits onto a work surface and cut into 2-inch squares. Brush the top and bottom of each grits cake with olive oil and arrange on a baking sheet. Top each with some of the pimiento cheese. Broil until golden, about 5 minutes on each side.