INGREDIENTS
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4 tablespoons (Â 1/2 stick) unsalted butter
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2 tablespoons olive oil
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 1/2 medium onion, chopped
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4 garlic cloves, chopped
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 1/4 cup chopped fresh parsley
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Kosher salt, freshly ground pepper
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1Â 1/2 cups dry white wine
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1 pound littleneck clams, scrubbed
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1 pound mussels, scrubbed, debearded
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3 tablespoons olive oil, plus more for drizzling
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 1/2 medium onion, chopped
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2 small leeks, white and pale-green parts only, quartered lengthwise, thinly sliced crosswise
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3 celery stalks, chopped
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4 garlic cloves, chopped
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 1/4 cup chopped fresh parsley
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Kosher salt, freshly ground pepper
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1 cup quartered peewee potatoes or baby Yukon Gold potatoes
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4 tablespoons (Â 1/2 stick) unsalted butter
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 1/4 cup all-purpose flour
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 3/4 cup chilled heavy cream
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 3/4 cup chilled whole milk
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3 ounces skinless, boneless smoked trout, coarsely flaked, divided
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Dill sprigs (for serving)