Preheat the oven to 200-210°C.
Roll out the shortcrust pastry and cover a tart pan with it. Using a fork, poke several holes in the base so that it does not swell during cooking.
Peel the onions and cut into julienne strips. Then, in a frying pan with a little olive oil, sauté the onions until tender.
While the onions are cooking, dice the zucchini. After the onions have been cooking for 10 minutes, add the zucchini to the pan, cover and cook for 5 more minutes. Add salt and pepper to taste.
Cut the salmon into cubes and season with salt and pepper. Reserve.
In a bowl, beat the eggs together with the yolks. Add the mascarpone cheese, cream, salt, pepper and chopped basil leaves.
On the tart base, spread the zucchini and onion mixture. On top, place the diced salmon and, finally, pour the egg and mascarpone mixture.
Bake in the oven for about 25-30 minutes, until the quiche is golden brown and the filling is set.
Let cool slightly before unmolding. Serve hot or warm.