Wash the spinach really well and then pat it dry with a dish towel.
In a very large skillet or Dutch oven, heat the olive oil and butter over medium heat. When the butter has melted, add the garlic and cook, stirring, for 1 minute.
Add the spinach, salt and pepper to the skillet, toss to coat, and then cover. Cook (covered) for about 2 minutes. Uncover and cook, stirring, for about 1 more minute while the liquid evaporates and the spinach wilts. Squeeze lemon juice over top, season with additional salt (if necessary) and serve with a slotted spoon.
Notes
You can use regular fresh spinach leaves or fresh baby spinach leaves for this recipe. If your leaves are particularly large, you might want to roughly chop them before sautéing.
I do not recommend using frozen spinach in this recipe.
What else can I add to the spinach? Cooked, chopped bacon is a nice addition to the dish. You can also try sauteed spinach with onions or tomatoes (diced fresh or canned diced will both work).
Instead of lemon juice, use balsamic vinegar for an acidic finish.