Lightly spray an 18 cm x 28 cm slice pan with oil and line with baking paper, extending paper 2 cm above pan edges. Cut floret tops from broccolini and thinly slice stems. Chop 1 zucchini into 1 cm pieces. Slice remaining zucchini lengthways into thin strips.
Heat oil in a non-stick frying pan over a medium heat. Add leek. Cook, stirring occasionally, for 5 minutes or until soft.
Add peas, garlic, chopped zucchini, broccolini stems and florets. Cook, stirring, for a further 3-5 minutes, until vegetables are just tender. Remove from heat. Cool slightly. Meanwhile, place zucchini strips in a microwave-safe dish. Cover and microwave on high (100%) for 1 minute or until just tender. Drain.
Whisk eggs in a large bowl. Season with pepper. Stir in vegetable mixture, basil and half the feta. Transfer mixture to prepared pan and spread evenly.
Arrange zucchini strips on top. Lightly press into mixture. Scatter over remaining feta. Bake in a 180 °C preheated oven for 40-45 minutes, until set and golden. Stand for 10 minutes, then use lining paper to lift frittata onto a chopping board. Cut into pieces. Serve with salad.