Ventilate your kitchen, then put a cast iron skillet on medium high heat. When the skillet is hot, place the scallions on the skillet and let them char, this will take about 2 minutes. Flip and let the other side char. The darker the better.
Chop off the stem ends from the scallions and add them to a food processor or blender.
Add in the basil, pecorino romano, pine nuts, lemon juice and garlic. Pulse until the mixture is chunky. Begin to gradually pour in the olive oil while the food processor is running, allowing the pesto to become a smooth paste.
If you are doing this in a blender, stream in the olive oil the same way, or add it in batches, about 2 tbsp at a time.
Season with salt and pepper to taste and serve!
This can also be store in an airtight container in the fridge for up to 7 days.