1. Crush garlic cloves with the flat of a chef's knife (do not peel). Crush coriander, cumin and peppercorns with a heavy saucepan. Tuck the garlic, crushed spices, red-pepper flakes and turmeric into a tea infuser (or cheesecloth bag) and place it in a large saucepan. Add cinnamon stick to the pan and pour in broth. Bring to a boil. Boil, uncovered, until reduced to 4 cups, 15 to 20 minutes.
2. Heat oil in a large heavy pot over medium heat. Add onions and cook, stirring, until softened and very lightly colored, 3 to 5 minutes. Add rice or bulgur and stir for 1 minute.
3. Pour in the reduced broth (including the tea infuser and the cinnamon stick) and bring to a simmer. Cover and simmer over low heat until the rice or bulgur is tender, 15 to 20 minutes.
4. Cut 4 slices from the lemon and reserve for garnish. Juice the remaining lemon to flavor the soup. Stir chickpeas into the soup and heat through. Remove the tea infuser and cinnamon stick. Season the soup to your taste with lemon juice and ground pepper. Ladle into bowls and garnish with lemon slices and mint.