Clean mussels by scrubbing their shells with a brush while rinsing under cold running water. If necessary, use a paring knife to tug and cut out weedy "beards" coming out of the bottom of the shells. Pick through mussels and discard any with broken or open shells that won't close when you tap them. Heat a large, dry skillet over medium-high heat. Add mussels, shallot, garlic and tomatoes and stir to combine. Cook for two minutes and then add beer and continue cooking and stirring until shells open, about another four minutes.
Transfer mussels to serving bowls with a slotted spoon and continue to simmer liquid to reduce it slightly, scraping up any bits stuck to the bottom of the pan with a wooden spoon. Remove pan from heat and swirl in butter. Stir in chervil or tarragon if using. Pour broth over mussels, sprinkle pretzel crumbs over each portion (if using) and serve immediately.
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