Place the spinach in a colander and make it wet under cool running tap water.
Working in batches and with only the water that adheres to the spinach, place it in a large nonstick skillet and heat the skillet over medium-high heat.
Cover the skillet to help wilt the spinach. When that batch is wilted, add more spinach and let that wilt until it's all wilted. Season the spinach with salt and black pepper to taste.
Transfer the spinach back to the colander to drain. Gently press on the spinach with the bottom of a ladle or 1-cup measuring cup to extract any residual water. Set aside.
In the skillet used to wilt the spinach, melt the butter over medium heat.
Add the shallot, reduce heat to medium-low and cook for 3-4 minutes or until the shallot is softened, being careful to not burn the shallot or brown the butter.
Add the garlic and cook briefly for 10-15 seconds.
Whisk in the flour and cook for 2 minutes, being careful to not brown the flour.
Add the half and half. Bring to a simmer over medium heat and cook for 1-2 minutes or until thickened.
Add the Boursin cheese and break it up to melt and get it incorporated into the thickened half and half.
Add the cayenne, nutmeg and Parmesan cheese. Stir until the Parmesan cheese melts.
Add the wilted spinach back into the creamy base and stir until it's evenly combined with the sauce. Add salt and black pepper to taste if desired. Serve immediately or cool and transfer to a baking dish to reheat at a later time.