Bring a saucepan of water to a boil and cook the fava beans for 3-5 minutes, then scoop out with a slotted spoon into a bowl of cold water to stop them from cooking. Put the peas into the water on the stovetop to cook for a few minutes while you squeeze the favas out of their skins into the bowl of a food processor.
Drain the peas, rinsing them under cool water too, and add them to the food processor with the lemon juice, garlic, yogurt, tahini, mint, dill, sumac and a big pinch of salt and good grind of black pepper. Add about 2 Tbsp of olive oil and blitz until relatively smooth. Taste and adjust the seasoning, adding more lemon if you like, or more olive oil to loosen it up. Serve drizzled with extra olive oil and a few mint leaves. Serves about 4.