Shell the fava beans from their pods. You should end up with about 400 g of beans.
Bring a 3-quart / 2.8L saucepan of well-salted water to a boil. Blanch the beans for 2 minutes, until the skins loosen and split slightly.
Drain and plunge the beans into a bowl of ice water for 1 minute to stop the cooking and set the color.
Once cool, pinch open each skin and pop out the bright green bean inside. Discard the skins. You should have about 300 g peeled beans. Skip this step if your beans are small and young, the skins will be tender enough to eat.
Heat the olive oil in a 10-inch / 26cm skillet over medium heat. Add the spring onion and garlic and cook for 2 minutes, until soft but not browned.
Add the peeled fava beans to the skillet and toss for 2 to 3 minutes, until warmed through and tender but still bright green.
Remove from the heat. Squeeze in the lemon juice and season with salt and pepper, tossing to coat.
Transfer to a platter, scatter the shaved pecorino and torn mint over the top, and finish with a drizzle of olive oil. Serve warm or at room temperature.