To make the dressing, trim and finely slice the spring onions, then place the green pieces in a large bowl. Quarter and add the tomatoes along with 6 tablespoons of oil, the zest from half the lemon and most of the juice. Season, then set the dressing aside. Squeeze the remaining lemon juice over the white part of the onions and set aside.
Pick and set aside the mint leaves, then tie the stalks together with string. Place the stalks and broad beans into a pan of boiling salted water, then blanch for around 2 minutes. Using a slotted spoon, transfer the broad beans to a bowl, discarding the mint stalks. Add the peas to the boiling water and blanch for 2 minutes.
Meanwhile, pod any larger broad beans and add to the dressing, then drain and add the peas. Finely slice and add most of the mint leaves, then toss everything well to coat.
Toast the sourdough in a griddle pan over a high heat. Halve the garlic and rub the cut side onto one side of the griddled bread, then rub over the rosemary. Sprinkle with a little salt and drizzle over some oil. Pile the peas and broad beans onto the dressed side of your toast. Sprinkle over the white spring onion and the reserved mint leaves, then grate the remaining lemon zest on top. Use a speed-peeler to shave the feta over the top, then serve.