In a large saute pan over medium heat, melt 8 tbsp. (1 stick) of butter. Add the onion and a teaspoon of salt and cook, stirring frequently until the onions are soft, about 3-5 minutes. Add the garlic, stir, and cook for 1 more minute until the garlic is fragrant but not burned. Reduce the heat to low and sprinkle the flour over the onions and garlic. Using a whisk, stir the flour to incorporate it into the butter and onion mixture. Cook over low heat for 2-3 minutes to eliminate the raw flour taste.
Whisking constantly, slowly add ¼ cup of the half-and-half to the onion mixture. The sauce will begin to thicken. Add another ¼ cup of the half-and-half, whisking completely to ensure there are no lumps (except for the onions).
When the sauce is smooth, add the rest of the half-and-half in a slow stream, whisking the whole time to incorporate it completely. Whisk in the Dijon mustard. Allow the sauce to simmer on low for 5 minutes or until it’s thick enough to coat the back of a spoon. Stir in the nutmeg, then taste, adding more salt and ½ teaspoon ground pepper if desired. Remove from the heat.
In a large nonstick frying pan, heat a teaspoon of butter over medium-high heat. Add 1/3 of the spinach and a pinch of salt. Allow it to sit for 15-30 seconds before using tongs to fold the spinach leaves that are on top down to the bottom of the pan. Cook the spinach for 1-2 minutes in total, or until it’s barely wilted. Move it to a bowl and repeat with the remaining spinach until it’s all wilted.
If there’s any liquid at the bottom of the bowl of spinach, drain it off before folding the spinach into the cream sauce. Taste, adding more salt, nutmeg, or pepper if desired. If using, fold in the Parmesan cheese.
Place the creamed spinach in a serving dish, top with additional Parmesan if using, and serve!