Cut the top off the head of garlic. Arrange the garlic, carrots, celery, onion, pepper, and tomato on a large baking sheet 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. Place the carrots, celery, onion, pepper, and tomato in the stock pot. Bring the water to a boil; reduce heat to low and simmer for 1 1/2 hours; strain and cool.
