Preheat the oven to 300ºF. Line a rimmed sheet tray with all of the vegetables. The vegetables should cover the tray in a single layer. (Note: the total weight of vegetables is about 4 lbs or 1.812 kg.) Drizzle olive oil over top. Season with 1 teaspoon kosher salt and pepper to taste. Roast for about three hours, but start checking after 2 hours — sometimes they are done in 2.5 hours. The vegetables should be soft and slightly caramelized.
Meanwhile, toast the bread. If you haven't already, slice the bread into ½-inch thick pieces. Place on the counter to dry or toast briefly in the toaster. You can also stick the bread in the oven for about 20 minutes or so while the tomatoes are roasting. You just want to dry out the bread; you’re not trying to brown it.
When the vegetables are done, place them in a pot with 2½ cups of water. Bring to a simmer. Note: It's best to bring this soup to a simmer slowly — it spits violently if you heat it too quickly. Also, reheat with the lid on over low heat for the same reason.
Season with a pinch of salt and crushed red pepper flakes if using. Add the bunch of basil. Break one slice of bread into medium-sized cubes and add to the pot. Using an emersion blender or food processor or traditional blender, puree the soup roughly. Add the other slice of bread if necessary. The soup should be slightly chunky. Taste and add more salt or bread if necessary. Thin with more water until soup reaches desired consistency.
Serve with a drizzle of olive oil and a few shavings of Parmigiano Reggiano.