Heat a cast-iron pan over medium-high heat. Once hot, add the oil. Add the zucchini in batches and sear on both sides until golden brown, about 1 minute per side. Once the second batch is seared, add the first batch back into the pan, and then add the thyme and garlic. Saute until the garlic is fragrant, another 30 seconds. Add the lemon juice, season with salt and pepper and stir to combine and heat through.