Cover a sheet pan with foil or parchment. Preheat oven to 275°.
Core the tomatoes and slice in half. Put the tomatoes on the sheet pan, cut side up. Drizzle with olive oil and season with salt and pepper. Roast for 2 to 2½ hours. Tomatoes with have shrunk considerably and the flavors will be concentrated.
While the tomatoes are cooking, slowly sauté the onions in a little olive oil over medium low heat until they are carmelized. This will take about half an hour. You can simply turn the heat off and let the onions rest on the stove until the tomatoes are ready. I use my soup pot to cook the onions so I don't mess up another pan.
When both the onions and tomatoes are done cooking, scrape them and any residual cooking oils into a food processor or blender and process until smooth. Add the tomato and onion purée to your soup pot. Add the Italian seasoning and dried basil. Whisk in the stock and bring to a simmer. Turn the heat to low and simmer for about 20 minutes.
Serve!