Place spinach in a large pot over high heat. Cook, covered, with just the water clinging to leaves, stirring occasionally, until wilted, about 2 to 4 minutes.
Transfer to a colander, immediately fill pot with cold water, transfer it back to the pot of cold water to shock it (stop the cooking) and drain again
Squeeze a small amount of the spinach at a time in your hands to extract as much water as possible. Chop the spinach coarsely. You should have about 3 cups of chopped spinach
Wipe out pot then melt 2 tablespoons butter over moderately high heat and stir in the spinach. Cook for 2 to 3 minutes or until all of the moisture from the spinach has boiled off — you’ll know you’re done when the spinach begins to stick to the pan
Lower the heat and sprinkle with flour and stir for 2 minutes to cook the flour. Add 2/3 of your stock or cream, a tiny bit at a time, scraping up any stuck spinach as you do. Once the liquid is added, simmer for another minute or two, stirring frequently to prevent sticking. Season with salt and pepper
Preheat oven to 375 degrees
Lightly butter a shallow 1-quart baking dish. Stir 1/2 cup cheese into the spinach and pour it into the baking dish. Mix the remaining cheese with breadcrumbs and sprinkle on spinach. Melt remaining butter and pour it over the top
Bake until heated through and slightly brown on the top, about 30 minutes