Preheat the oven to 400 degrees Fahrenheit.

Either wrap the whole beets together tightly in aluminum foil or place in a lidded cast-iron skillet (rubbed with a bit of oil). Roast the beets for 45 minutes, remove from the oven, unwrap or uncover, and allow to cool until they can be easily handled.

Slip off the skins and slice each beet into 1/4-inch rounds, then slice the rounds into half moons.

Place the sugar, salt, water, and vinegar into a small saucepan and set over medium heat. Cook until the sugar and salt have dissolved, then add the dill seeds and cook for one minute longer. Remove from the heat.

Take a clean, one-quart canning jar and alternate laying the beet and the onion slices; tuck the fresh dill weed in there, too. Carefully pour the dill brine into the filled jar covering the beets by half inch. Place the lid on the jar and allow to cool at room temperature, then store in the fridge for up to three weeks.