Heat olive oil in a large saucepan on the grates of the grill. Add the onions and garlic and cook until soft. Add chickpeas, stock and salt and pepper and cook until chickpeas are very soft. *Transfer mixture carefully, in batches, to a blender and blend until smooth. Return the mixture to the saucepan and bring to a boil. Slowly whisk in the polenta, stirring constantly, and continue whisking until mixture begins to bubble. Reduce heat to medium and continue cooking, stirring with a wooden spoon until the mixture is thickened and cooked, about 30 minutes. Season with salt and pepper, to taste.
Spread the mixture into a baking sheet, cover with plastic and place in the refrigerator until cold and very firm, about 4 hours. When the polenta is firm, remove from the refrigerator and, using a 2-inch ring mold, cut out circles of the polenta.
Preheat a grill to medium-high.
Season the polenta circles with salt and pepper on both sides, brush with oil and grill on both sides until golden brown, about 2 to 3 minutes per side. Remove from grill, drizzle with chive oil and garnish with lemon zest and chives.