Preheat the oven to 350 degrees.

Cut the tomatoes and onion into wedges, then spread on a rimmed baking sheet. Drizzle with the olive oil, salt, pepper, and chopped parsley. Gently toss together with your hands. Tuck the garlic cloves into a tomato so that they don't burn.  Roast for approximately 40 minutes until the tomatoes are reduced to about half their size. Remove and let cool.

Warm the vegetable stock in a large pot on medium heat, and stir in tomato paste.  Add the ingredients from the roasting pan into the broth and simmer for 8-10 minutes.  Use an immersion blender to puree the soup in the pot, or transfer to blender or food processor and blend until soup is smooth.  Add any more salt and pepper to taste.