In a saucepan of boiling water, cook the fava beans until the skins start to loosen, 1 1/2 minutes. Drain and squeeze out the favas. Transfer the favas to a food processor and add the oil, lemon juice and zest and pulse to a coarse puree. Season with salt and pepper.
Spread the fava-bean puree on the toasts and top with the mozzarella strips. Drizzle the toasts with the balsamic vinegar, scatter the mint on top and serve.