Lightly oil a 9-inch round glass baking dish. In a medium saucepan, bring 3 cups of water to a boil. Add 1 1/2 teaspoons of salt and slowly whisk in the polenta. Cook over low heat, whisking constantly, until the polenta is thick, about 5 minutes. Pour the hot polenta into the baking dish and cover the surface directly with plastic wrap. Let stand until firm, about 25 minutes.
In a large, deep skillet, heat the 2 tablespoons of olive oil. Add the garlic and cook over moderate heat until golden brown, about 30 seconds. Increase the heat to moderately high and add the spinach, stirring to wilt it. Season with salt and pepper. Keep the spinach warm.
Light a grill and oil the grates. Cut the polenta into 8 wedges and grill over moderately high heat until crisp and lightly charred, 5 minutes per side. Place 2 wedges on each plate; top each with 2 robiola slices. Spoon the warm spinach on top and serve.