The incredible mound of spinach called for here (3 pounds!) deflates quickly after an initial blanching, which helps maintain its deep green color. Shallots and garlic lend sweetness, while a mix of milk and heavy cream provides richness without being too heavy. Finally, tangy sour cream lifts and balances the dish. The spinach can be blanched, cooled and chopped a day ahead and stored in the refrigerator. Leftovers make a great omelet or quiche filling, and are also a terrific addition to pasta.
In a large pot, bring ½ inch of water to a boil over high. Add spinach in 3 batches, stirring until each batch is wilted before adding another, until all of the spinach is wilted, about 3 minutes. Drain in a colander and run under cold water until cool. Drain again, pressing out excess water, then coarsely chop for a total of 4 packed cups.
In the same pot, melt 2 tablespoons of the butter over medium-low. Add shallot and cook, stirring, until softened with no color, about 2 minutes. Stir in garlic until fragrant, 30 seconds. Add the remaining 1 tablespoon butter and stir until melted, then add flour and cook, stirring constantly, until flour is light golden but the shallots still have no color, about 3 minutes. While whisking constantly, drizzle in milk and cream until smooth, and season with salt and pepper. Bring to a simmer, then add chopped spinach and season with salt and pepper. Cook, stirring, until the spinach is totally incorporated and the sauce is thickened, 2 to 3 minutes. Stir in sour cream until combined. Transfer to a bowl and serve warm.