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Place the onion and butter in a medium saucepan and cook, covered, over medium heat until the onion is soft but not browned. It will take about 10 minutes. Stir a few times.
When the onion is soft, add the baby spinach and peas and cook, stirring, for the few minutes it takes for the spinach to just wilt and the peas to warm through.
Remove from the heat and puree to a rough mash using a stick blender (or transfer to a food processor).
Add the stock powder (if using) and vinegar. Stir well. Taste and season with salt and pepper.