Heat olive oil over medium heat in a large skillet. Add the spinach (in batches if necessary) and cook until just wilted. Remove from skillet and set aside in a colander to allow any extra liquid to drain.
Add the butter to the skillet with the diced shallot. Sauté until tender, just a few minutes. Add the garlic and continue cooking until fragrant, about 30 seconds to 1 minute.
Sprinkle in the flour and cook, stirring, about 1 minute to remove the flour taste.
Slowly pour in the milk and the cream, and simmer very gently, stirring, until thickened to your liking.
Season to taste with salt and pepper and stir in wilted spinach.
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