1. Warm the olive oil in a large pot on medium heat. Add the chopped onion and cook for 5-6 minutes until softened and translucent. Add the diced carrots, celery, and bell pepper and sauté for an additional 4-5 minutes until they start to soften.
2. Stir in the parsley, basil, thyme, dill, salt, and flour, making sure to scrape up any flour that may have stuck to the bottom of the pot. Gradually pour in ½ cup (120 ml) of milk while continuously whisking, ensuring that all the flour dissolves. A flat whisk is ideal for this task, but a balloon whisk will suffice. Continue whisking until the mixture is smooth.
3. Add the remaining milk, vegetable broth, and nutritional yeast (if using). Combine well. Increase the heat to high and bring the mixture to a boil. Once boiling, add the pasta and reduce the heat to medium-low. Simmer for 10-15 minutes, stirring regularly to prevent the pasta from sticking, until it's cooked through.
4. Taste and adjust seasonings to your preference.