Dice the onion and mince the garlic. Add the onion, garlic, and butter to a large soup pot. Sauté over a medium flame until the onions are soft and transparent (about 5 minutes).
Add the flour to the pot and continue to stir and cook for 1-2 minutes. The flour will form a paste with the butter and onions and begin to coat the bottom of the pot. As soon as the flour that is stuck to the pot begins to turn slightly golden, remove it from the heat.
Add the crushed tomatoes (with juices), roasted red peppers (without juices) and the flour paste from the soup pot to a food processor or blender. Blend until smooth, then return the contents to the soup pot.
Add the vegetable broth, basil, thyme, and some freshly cracked pepper (10-15 cranks of a pepper mill) to the soup and stir to combine. Heat and stir over a medium flame until the soup begins to simmer. Use the spoon to help dissolve any flour stuck to the bottom of the pot as you stir.
Once the soup begins to simmer, turn off the heat. Add the milk, if using, and stir to combine. Taste and add salt if needed (this will depend on the salt content of your broth).
NOTES
*I use Better Than Bouillon soup base to make my broth.