Pack the green tomatoes into a wide-mouth mason jar and place the sliced jalapeño on top.
Heat a small saucepan over medium heat and add the mustard seeds. Cook for 1 minute or so until they begin to toast and pop. Add the peppercorns and cook for 30 seconds more.
Carefully pour in the water, vinegar, sugar, and salt and bring to a boil. Boil for 3-4 minutes until the sugar is completely dissolved. Turn off the heat.
Carefully pour the hot liquid into the jar and ensure that the tomatoes are completely immersed. If necessary, top off the jar with a bit of warm water.
Cover and transfer to the fridge to chill for at least one hour before serving.
Enjoy the pickled green tomatoes on top of salads, rice bowls, or tacos. If kept in an air-tight mason jar, pickled green tomatoes will last for about a month in the fridge.