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.
In a very large pot or Dutch oven, heat the olive oil and saute the garlic over medium heat for about 1 minute, but not until it's browned. 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. Using a slotted spoon, lift the spinach to a serving bowl and top with the butter, a squeeze of lemon, and a sprinkling of sea or kosher salt. Serve hot.