Give the jalapenos a good rinse and then cut them into 1/4 inch slices, discarding the stems.
Roughly chop the peeled garlic clove and a few tablespoons worth of onion.
Add 1 cup white vinegar, 1 cup water, and 4 cups white sugar to a saucepan. Bring to a boil, then reduce heat and let simmer for a few minutes.
Saute the onion and garlic in a glug of oil over medium heat using a skillet large enough to hold the jalapenos. Once the onion has softened, add the spices and salt: pinch of cumin, pinch of Mexican oregano, black peppers, and 1/2 teaspoon salt. If using seeds and peppercorns I usually give them a quick crush in the molcajete but this is optional.
Cook the spices for a moment and then add the sliced jalapenos and the vinegar mixture. Bring to a boil, then turn down the heat and let the jalapenos simmer for a couple minutes.
Add the mixture to a quart-sized Mason jar (or two pint-sized jars). I usually use a slotted spoon or ladle to add the jalapenos first, and then pour the brine on top of them. Partially cover and let sit on the counter for a few minutes to cool down.
Once cooled you can cap the jar and store it in the fridge. They'll have most of their flavor the next day, but may continue to sweeten up over the coming days. They'll keep for up to a month in the fridge.