Preheat the oven to 450 degrees F (230 degrees C).

Place onions, tomatoes, carrots, bell peppers, and turnips in a large roasting pan; drizzle with olive oil and toss to coat.

Roast in the preheated oven, stirring every 15 minutes, until vegetables are tender and onions are caramelized, about 1 hour.

Transfer roasted vegetables into a large stockpot. Add celery, parsley, garlic, peppercorns, cloves, and bay leaf. Pour in water and bring to a boil over high heat. Reduce heat to medium-low and simmer, uncovered, until liquid is reduced by half, 20 to 40 minutes.

Strain broth into a large bowl, reserving vegetables for another use.