In a large bowl add the onions, tomatoes, carrots, green peppers, and turnips and drizzle with olive oil and stir well to ensure well coated. Place veggies onto a large baking sheet.
Roast in the oven stirring veggies every 15 minutes, until vegetables are tender and onions are caramelized, about 50 minutes.
In a large stock pot add the roasted vegetables, water and the rest of the ingredients.
Bring to a low simmer over medium high heat. Reduce heat to medium-low and continue to simmer, uncovered, until liquid is reduced by almost half. About 30 to 40 minutes.
Pour the vegetable broth through a cheesecloth or fine mesh strainer into a large bowl. You can eat the vegetables or put them in a freezable container for another use.
Notes
Feel free to also use clean onion skins, celery leaves and clean unpeeled carrots as these are also great for adding flavor to your vegetable broth.