Arrange the tomatoes, red pepper, yellow onion, and garlic on a sheet pan. Sprinkle the oregano, parsley, salt, and pepper evenly over the veggies. Drizzle the olive oil evenly over the veggies, and toss everything together with your hands.
Roast the veggies for 40 minutes, until the veggies look shriveled (and the tomatoes are cracked on the skin) and soft.
For the Parmesan Croutons:
Reduce the oven to 375F.
Toss the bread squares in the olive oil and spread them on a baking sheet lined with parchment paper. Parchment is optional if your baking sheet doesn't have an issue with food sticking. Sprinkle the bread with a pinch of salt. Bake until the bread is crispy, or about 15 minutes.
Sprinkle the cheese over the crispy croutons. Gently toss to cover the croutons with the cheese (you'll probably need a large spoon-they'll be warm to touch).
For the Soup Base:
Add the broth and tomato paste to a soup pot (or Dutch oven) on the stove-top. Whisk to combine the broth and paste. Over medium-high heat, bring the broth to a slight boil. Once the broth is warm, add the roasted veggies and basil.
Use an immersion blender to puree the ingredients into a blended soup (there will always be some little pieces in the soup). You could use a high-speed blender instead of an immersion blender for a smoother soup. The soup should be hot at this point, but if not continue to cook it on the stove-top until it's hot.
Finally, stir in the milk. The milk helps balance the acidity of the tomatoes.
Taste the soup and add more salt, if needed.
Serve the Soup:
Spoon the soup into a bowl(s). Add a few parmesan croutons to the top of the soup. Garnish with extra chopped basil and black pepper, if desired.
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Notes
As an alternative to the croutons, make homemade parmesan crisps. This is a delicious, low-carb, gluten-free way to enjoy a crunchy, salty topping for the tomato soup. Make homemade parmesan crisps.