Preheat the oven to 400 degrees F (200 degrees C).

Cut top off head of garlic. Arrange garlic, carrots, celery, onions, bell pepper, and tomato on a large baking sheet in a single layer; drizzle with olive oil, then season with salt and black pepper.

Roast in the preheated oven, turning every 20 minutes, until tender and browned, about 1 hour.

Combine water, bay leaves, thyme, and parsley in a large stock pot. Squeeze head of garlic into stock pot; discard outer husk. Add carrots, celery, onions, bell pepper, and tomato to stock pot. Bring to a boil over medium-high heat; reduce heat to low and simmer 1 1/2 hours; strain and cool.