Heat the olive oil over medium-high heat or until the pan reaches 320°F. Add the chopped onions and sauté until the onions are just starting to brown, about 15 minutes. Season with salt and pepper and continue sautéing, stirring occasionally, until the onions are deeply browned and sweetened – about one hour.
Once the onions have caramelized, add the peach syrup, brown sugar, honey, ground nutmeg, ground cloves, allspice, and the cinnamon stick to the pot. Stir well to combine.
Place the peaches into the pot of a pressure cooker and pour the onion sauce over the peaches so they are completely coated. Pressure cook the peach mixture for 10 minutes.
After pressure cooking, carefully transfer the peaches to glass mason jars and allow them to cool to room temperature.
Serve the Sploosh as a refreshing drink, pick out the peaches for a snack, or use it as a flavorful marinade for your favorite meats. Enjoy!