To make the chicken: Place the chicken breasts in a slow cooker. In a small bowl, combine the onion, garlic, oregano, salt, pepper, olive oil, red wine vinegar, lemon juice, and water. Pour over chicken. Cook on low for 6-8 hours or high for 4-6 hours. Once the chicken is cooked, shred using two forks.
To make the tzatziki: In a medium bowl, combine Greek yogurt, cucumber, garlic, dill, and oregano and lemon juice. Season with salt and pepper, to taste. Refrigerate for 30 minutes to allow the flavours to blend.
To make the potatoes: preheat the oven to 375 degrees F. Place the mini potatoes, olive oil, minced garlic, salt and pepper in a large bowl and toss to combine. Distribute the potatoes evenly on a baking sheet. Roast in oven for 45 minutes to an hour, or until soft when pierced with a fork.
To assemble the stuffed mini potatoes: Cut each mini potato down the centre, approximately 3/4 of the way down, without slicing all the way through. Evenly distribute the shredded chicken among the potatoes, inserting it into the area that was cut. Top each stuffed potato with a dollop of tzatziki and garnish with a sprig of fresh dill or parsley, if desired.