Cut the top off the head of garlic. Arrange the garlic, carrots, celery, onion, capsicum and tomato on a large baking tray in a single layer. Drizzle the olive oil over the vegetables; season with salt and pepper.
Roast the vegetables in the preheated oven, turning every 20 minutes, until tender and browned, about 1 hour.
Combine the water, thyme, parsley and bay leaves in a large stock pot over medium-high heat. Squeeze the head of garlic into the stock pot, and discard the outer husk. Add the remaining roasted vegetables to the stock pot. Bring the water to a boil; reduce heat to low and simmer for 1 1/2 hours; strain and cool.