Mix the flour, seed mix and salt in a bowl. Add the sugar, yeast, butter and water. Knead to form a soft, moist dough using the dough hook on a food processor or hand mixer. Cover and leave to rise at room temperature for approx. 1 1/2 hrs.
To shape Transfer the dough to the prepared tin. Press the dough firmly into the corners. Cover and leave to rise again at room temperature for approx. 15 mins.
Bake for approx. 30 mins. in the lower half of an oven preheated to 220°C. Remove from the oven, brush with a little water, allow to cool slightly, tip out onto a rack and leave to cool.
Combine the vinegar and oil, add the onion, season.
Depending on their size, chop or slice the mushrooms. Heat the oil in a non-stick frying pan. Fry the mushrooms in batches for approx. 3 mins. per batch, remove, season.