Heat oil in medium pot on grates of grill or side burners. Add onions and garlic and cook until soft. Add the rice and stir to coat the grains of rice with the oil.
Place the water in a small pot and bring to a boil. Add the saffron to the onion mixture and let boil 1 minute. Add the boiling water to the rice, stir, add salt and pepper and bring to a boil. Reduce the heat to medium-low, cover the pot and let cook 12 to 14 minutes or until the rice is just cooked. Let the rice sit, covered for 5 minutes then fluff with fork.
Transfer the rice to a large bowl and add the asparagus, peppers, olives and herbs. Drizzle with extra-virgin olive oil and a few tablespoons of aged sherry vinegar. Serve warm or at room temperature.