Rinse the mussels in cold water. Remove any broken mussels or mussels that are open and won't close when you give them a pinch. If some of the mussels have large "beards" pinch them off as shown in the picture. Let the cleaned mussels soak in cold water while you prepare the rest of the ingredients.
Open the can of whole tomatoes and place the tomatoes and their juice in a bowl.
Use clean hands to "squish" the whole tomatoes until you achieve a chunky, rustic tomato sauce. Measure out 3/4 cup of this sauce and save the rest for another use.
Mince 1/3 cup shallot
Finely chop 1/3 cup garlic
I am using calabrian chili that is already chopped. If you have whole calabrian chilies, remove the stems and seeds, chop the chilis and mix the chopped chili with a little bit of the chili oil. You can also substitute calabrian chili with a big pinch of red pepper flake)
Drain the mussels well of the water they were soaking in. Heat 1.5 Tbsp oil in the large pot over very high heat. The burner should stay on maximum heat for the entire cooking process.
Let the oil heat up until it is smoking. Add the garlic and shallot and cook while stirring until the garlic just starts to brown. This will only take a few seconds.
Immediately add the mussels to the pot to stop the garlic from burning. Give the mussels a good stir and sprinkle on a generous pinch of salt and pepper.
Immediately after stirring, add the canned tomato and the calabrian chili. Push the tomato and chili to the bottom of the pot to let them cook in direct contact with the bottom of the pot for about 30 seconds. (If you are unsure of how spicy the chilis will be, add only 1/2 Tbsp now and then add the other 1/2 Tbsp at the end, after tasting the broth to ensure it isn't too spicy)
After cooking the tomato and chili for 30 seconds, add the chicken stock and butter. Stir well and allow to cook until the butter has melted.
Once the butter has melted, most of the mussels should already be open, indicating that they are done cooking. If they are still closed, cover the pot and cook just until the mussels have opened. Don't cook the mussels longer than is necessary to open them.
Once all of the mussels are open, add the lemon juice and parsley and give the mussels a stir. Now the important step... taste the mussel broth and adjust the consistency and seasoning. The broth should have a soup consistency, without being overly thin. If it is very thick, add a touch more chicken stock. If it is too watery, let it cook a bit longer, uncovered, to reduce. The broth will likely need more salt and possibly a bit more lemon juice. Taste the broth and adjust as necessary. For reference, I used about 2 full teaspoons of kosher salt for this recipe.
Serve in a bowl or directly from the pot with some grilled or toasted bread.