Heat the 1 tablespoon extra-virgin olive oil in a large, deep skillet over medium heat.
Add the 2 small shallots and 5-6 garlic cloves and cook for 2-3 minutes, until just softened.
Whisk the 2 tablespoons cornstarch flour and 3/4 cup whole milk until smooth with no lumps.
Gradually pour in the milk, whisking as you go. Add the 1/4 cup heavy cream, ½ teaspoon Dijon mustard, ¼ teaspoon ground nutmeg, ½ teaspoon fine sea salt, ¼ teaspoon freshly ground black pepper, and ¼ teaspoon red pepper flakes.
Bring to a gentle simmer and cook for 2-3 minutes, stirring frequently, until the sauce thickens.
Add a big handful of the 1 pound fresh baby spinach and stir into the sauce until just wilted. Continue adding the fresh spinach in batches, stirring after each addition. As one batch wilts down, add the next.
Once you've added all the spinach, stir in the 1 tablespoon fresh lemon juice and 1 teaspoon finely grated lemon zest. If you'd like it even thicker, sprinkle with 1 tbsp cornstarch and stir well to combine.
Cook for another 2-3 minutes, stirring occasionally, until tender and the sauce has thickened. Don't overcook it, it should stay bright green!
Reduce the heat to low. Stir in the ½ cup finely grated Parmesan cheese until melted and the sauce is creamy and glossy.
Taste and adjust the salt, pepper, or nutmeg. Let the creamed spinach rest for 2-3 minutes before serving, as the sauce will continue to thicken slightly.
Serve warm with a little extra Parmesan and freshly cracked black pepper.
Notes
Serving recommendation: 6 servings, about ½ cup each
Use 5 garlic cloves for a milder flavor or 7 for a stronger garlic flavor.
Dry washed spinach well so it doesn’t water down the sauce.
Add spinach in batches. It will shrink considerably as it wilts.
For a thicker sauce, first let the dish rest for 2 to 3 minutes. If needed, mix up to 1 tablespoon additional cornstarch with 1 tablespoon cold milk or water. Stir in a little at a time and simmer briefly until thickened.
Store leftovers in the refrigerator for up to 3 days. Reheat gently with a splash of milk if needed.