Start the garlic and oil in a cold skillet to infuse the oil with flavor and create tender sweet caramelized garlic slices with each bite of the cooked spinach.
Add the olive oil and slivered garlic slices to a cold, large skillet and bring to medium heat. Cook until the garlic is fragrant and begins to caramelize, but watch it closely so it doesn't brown or it will become bitter. Quickly remove the garlic from the oil with a mesh skimmer and reserve the oil in the skillet.
Add the spinach in heaping batches, tossing in the oil as it reduces. Season with the kosher salt and freshly ground pepper and cook just until it wilts. Add the garlic back to the spinach, toss, and drizzle with the lemon juice or vinegar and taste for seasoning. Serve hot.