Cut the bunched onions in about 3 cm long pieces. Halve any large onion bulbs lengthways. Halve the courgettes lengthways and cut in thin 5 mm slices. Heat oil in a frying pan. Add the courgettes, onions, peas and sugar snap peas, stir-fry for 5 minutes over a high heat. Stir from time to time, season with salt and pepper, leave to cool.
Grate the Emmentaler AOP on a coarse grater. Place the pastry on a baking tray, pierce in many places with a fork. Brush the pastry with sour cream, distribute the vegetables over the top and sprinkle over the grated Emmentaler AOP. Bake on the lowest rung of a 200 degree hot oven for 20-30 minutes.
Garnish the tart with rocket, cut into pieces and serve.