1) Rinse the spinach well in cold water to make sure it's very clean. Spin it dry in a salad spinner, leaving just a little water clinging to the leaves.
2) In a very large pot or casserole dish, heat the olive oil and saute the garlic over medium heat for about 1 minute, but not until it's browned.
3) Add all the spinach, the salt, and pepper to the pot, toss it with the garlic and oil, cover the pot, and cook it for 2 minutes. Uncover the pot, turn the heat on high, and cook the spinach for another minute, stirring with a wooden spoon, until all the spinach is wilted.
4) Using a slotted spoon, lift the spinach to a serving bowl and top with the butter, a squeeze of lemon, and a sprinkling of salt.
5) Serve hot.