1. Blanch and drain the spinach: Rinse the spinach well and place it, still damp, in a large pot or deep skillet over medium heat. Cook, tossing often, until the leaves are fully wilted and reduced in volume.

2. Squeeze out excess liquid: Transfer the wilted spinach to a colander and let it cool slightly, then press firmly with the back of a spoon or squeeze in a clean kitchen towel to remove as much water as possible. Chop the drained spinach into smaller pieces if the leaves are large.

3. Sauté the aromatics: In the same pan, melt the butter with the olive oil over medium heat. Add the diced onion or shallot and cook until soft and translucent, 4 to 5 minutes. Stir in the garlic and cook just until fragrant, about 30 seconds.

4. Make the roux: Sprinkle the flour over the onion mixture and stir until everything is coated. Cook, stirring constantly, for 1 to 2 minutes to remove the raw flour taste without letting it brown.

5. Build the cream sauce: Slowly pour in the milk while whisking to avoid lumps, then add the heavy cream. Continue to cook, stirring often, until the sauce thickens and lightly coats the back of a spoon, 3 to 5 minutes.

6. Season the sauce: Stir in the salt, black pepper, nutmeg, and red pepper flakes if using. Add the grated Parmesan cheese and stir until it is completely melted and the sauce is smooth and glossy.

7. Add the spinach: Fold the drained spinach into the cream sauce, breaking up any clumps with a spatula or tongs, until every leaf is evenly coated. Let the mixture simmer gently for 2 to 3 minutes so the flavors meld and the spinach is heated through.

8. Adjust and serve: Taste and adjust the seasoning with more salt, pepper, or Parmesan as needed. Serve the creamed spinach hot as a side dish with steak, chicken, pork, or your favorite main course.