Place the minced onion in a small bowl and cover with cold water, adding one ice cube. Allow the onion to soak for a minimum of 5 minutes to mellow its flavor. Drain thoroughly, pat dry with paper towels, and set aside.

Prepare an ice bath by filling a medium bowl with ice water and set aside.

In a medium saucepan, bring generously salted water to a boil. Reduce the heat to a gentle simmer and add the shrimp. Cook for 5 to 7 minutes, or until the shrimp are fully pink and firm to the touch.

Using a slotted spoon, immediately transfer the shrimp to the prepared ice bath to halt the cooking process. Once cool enough to handle, peel and devein if necessary. Pat the shrimp dry and transfer to a cutting board.

Using a sharp knife or food processor, chop the shrimp until very finely minced, nearly forming a paste. Set aside.

In a large mixing bowl, beat the softened butter with a wooden spoon until smooth and creamy.

Add the chopped shrimp, soaked onion, kosher salt, lemon zest, lemon juice, Worcestershire sauce, parsley, dill, black pepper, and celery seed to the butter. Using the back of a fork or the wooden spoon, mash and mix until all ingredients are evenly incorporated.

Taste and adjust seasoning as needed.

Serve immediately with saltines or other light, crisp crackers. Alternatively, transfer the shrimp butter to an airtight container and refrigerate for up to one week. Before serving, allow it to soften at room temperature for at least 30 minutes, then re-beat until smooth and spreadable.

For longer storage, spoon the shrimp butter onto plastic wrap, shape into a log, and freeze for up to one month. Thaw in the refrigerator, then bring to room temperature and re-beat before serving.