Pre heat the oven to 180 deg C.
Bring a large pan of water to the boil to blanch the asparagus. To prepare the asparagus trim off the woody ends, keep the stalks the same length. Blanch in the boiling water for 1 minute then cool under cold running water. Set aside.
Remove the stalks from the portobello mushrooms and peel off the skin, place onto a tray. Season with a pinch of salt and pepper and place a small knob of butter onto each mushroom. Place into the oven for 10 minutes to cook. Remove and set aside.
Cut a quarter off the pastry sheet. Use the cut quarter to edge the pastry: cut this piece into 1cm strips. Brush the tart base with the egg wash, frame the base piece with the 1cm strips. Brush the edges with egg wash. Using a fork dock holes into the base pastry piece.
Spead the cooled mushroom duxelles over the pastry, leaving the edges clean. Arrange the blanched asparagus, mushrooms and cherry tomatoes how you like.
Place into the pre heated oven and bake for 25 minutes or until golden and crisp. Remove from the oven and crumble over the feta and garnish with fresh basil leaves.
Serve warm or at room temperature.