Brush a deep 20cm loose bottomed cake tin with a little of the oil. Keeping your filo pastry covered whilst you work, brush the sheets with oil and use to line the tin, carefully pushing it down to sit against the sides and base. You will need to overlap them and leave overhang at the top to cover the pie. Reserve 1 sheet of filo. Sprinkle the semolina or polenta over the base then begin layering your vegetables, pushing down each layer gently to make it level and ease out any air gaps. Season with a little salt and pepper as you go. Finish with the basil and mint leaves. Fold the overhanging filo sheets on top ensuring the pie is covered and brush with oil. You can cover and chill the pie at this point overnight.