In a large pot or Dutch oven, heat the olive oil over medium heat. Add the onions and cook, stirring occasionally, until golden brown, about 15 minutes. Add the garlic and cook, stirring, for 1 minute more. Stir in the chicken stock, tomatoes and their juices, saffron, 1 tablespoon salt (or, if you prefer, start with 2 teaspoons), and pepper. Bring the soup to a boil and then lower the heat and gently simmer for 15 minutes.