Wash the spinach thoroughly and pat it as dry as possible. Slice half of the lemon into thin rounds and set the other half aside for juicing later.
Warm the olive oil in a large skillet or Dutch oven over medium-low heat. Place the lemon slices in the oil in a single layer.
Cook the lemon slices for 2-3 minutes, flipping once, until they are fragrant and lightly golden. Use tongs to remove the slices to a plate, leaving the flavored oil behind.
Add the sliced garlic to the skillet and cook for 30-60 seconds, stirring constantly, until fragrant. Do not let it brown.
Add the spinach to the pan in large handfuls. Use tongs to toss the spinach as it wilts, which will make room for the next handful. Continue until all spinach is in the pan.
Increase the heat to medium and cook, tossing occasionally, until all the spinach is just wilted, about 2-4 minutes.
Remove the skillet from the heat. Carefully tilt the pan to drain off any excess liquid.
Season the spinach with salt and pepper. Squeeze the juice from the reserved lemon half over the top and toss to combine.
Transfer the spinach to a serving dish. Garnish with the grated Parmesan cheese and the reserved cooked lemon slices. Serve immediately.