Slice the onions and allow them to soften the in olive oil with the garlic, thyme, bay leaf, salt and pepper. Cook it gently for at least one hour.
Roll out the shortcrust pastry. Cook it for 10 minutes at Gas Mark 5/6.
Spread a fine layer of onions over the pastry. Arrange the porcini mushrooms in thin slices and place them on top of the onions. Season to taste with salt and pepper. Pour a drizzle of olive oil over the tart and put in the oven for approx. 5 mins.