For the tomato soup: Heat a large saucepan over medium heat and add the oil. When hot, add the onion and a large pinch of salt and cook until starting to soften, about 2 minutes. Add the garlic and cook for an additional minute. Mix in the tomatoes and chicken stock. Add the cream, hot sauce and oregano if using and bring to a simmer. Simmer until thickened and the flavors come together, 20 to 30 minutes. Taste and season with additional salt if needed. Let cool slightly, then transfer to a blender and puree until smooth.
For the grilled cheese: Heat a large skillet or griddle over medium heat.
Spread the mayonnaise or melted butter on one side of each slice of bread. Sprinkle the Parmesan over the mayonnaise or butter. Flip the slices and put 4 pieces of Cheddar on 4 of the bread slices. Spread the other bread slices with mustard, if using, and close the slices together to form sandwiches (the mayonnaise and Parmesan should be on the outside).
Working in batches if necessary, transfer the sandwiches to the skillet and cook until browned and toasted, 2 to 3 minutes per side. Slice and serve with the tomato soup.
For more ingredient substitution ideas, see the article below.
When blending hot liquid, first let it cool for five minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters, and pulse until smooth.
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