In a small bowl, combine 1 tablespoon of butter with the flour, mixing until fully blended into a smooth paste. Set aside. This mixture will serve as the thickening agent for the cream sauce.

In a large 12-inch skillet set over medium heat, melt the remaining 2 tablespoons of butter. Add the finely chopped onions and cook for 3–4 minutes, stirring occasionally, until they soften and become translucent.

Add approximately one-third of the spinach leaves to the skillet, ensuring that the pan is not overcrowded. Sprinkle with salt and pepper, then pour in 2 tablespoons of water. Using tongs, turn the spinach frequently until it begins to wilt. Transfer the wilted spinach to a bowl, leaving some liquid in the skillet. Repeat this process with the remaining spinach until all is cooked and wilted. Remove the pan from heat once complete.

Tilt the bowl containing the spinach and drain any residual juices back into the skillet. Set the spinach aside. To the skillet, whisk in the heavy cream, the prepared beurre manié, and the Dijon mustard. Return the pan to medium heat and bring the mixture to a gentle boil, whisking constantly until the sauce thickens and achieves a smooth consistency. Taste and adjust seasoning with additional salt and pepper if necessary.

Fold the wilted spinach into the prepared cream sauce, stirring until the leaves are evenly coated and the dish is heated through. Serve immediately as a side to your preferred main course.