Wash a wide-mouth pint mason jar and and its lid in hot soapy water. Then rinse and let air dry.
Trim the green beans so they fit in jar with about 1/2” of space remaining at the top.
Add the green beans, garlic and bay leaf into the jar, squeezing them in as tightly as possible. (Add the fresh dill sprigs if using.)
In a small saucepan, combine the white vinegar, water, sugar, kosher salt, peppercorns, and dill seed if using. Bring to a boil, stirring occasionally to dissolve the sugar and salt (this will take only a few minutes).
Once boiling, pour the brine into the jar. Tap the jar on the counter to release any air bubbles. Discard any remaining brine, or top off the jar with extra water if any green beans are exposed at the top.
Screw on the lid tightly and allow to cool to room temperature (about 1 hour). Refrigerate 24 hours for best flavor. Keeps up to 1 month refrigerated.