Spinach cooks down a great deal, but you'll need a BIG pot to hold it at first. One of those blue-speckled enamel pots that we used for crab-boiling when I lived in Baltimore would do fine. Or cook the spinach in batches and recombine in the serving bowl.
Remove stems and roots from spinach. Rinse in cold water many times until any trace of sand is removed. Place in a large pot with 1/4 tsp salt and no water other than that which clings to the spinach from washing. Cook over moderately high heat, covered, until desired tenderness. Transfer to a colander to cool, lifting the spinach out of the pot rather than pouring it, so that any sand remains in the pot.
Turn onto a cutting board and chop very fine. Do not use a food processor.
Place in a serving dish and flatten with a spatula. Sprinkle with the remaining salt and the oil. Just before serving, sprinkle with lemon juice.