Place the sheet of puff pastry on a tray lined with baking paper and score a border 2cm in from the edge all the way round. Prick the pastry centre using a fork. Leave to rest in the fridge for about an hour.
Meanwhile, preheat the oven to 220˚C with an empty baking sheet in it to get hot.
When the oven reaches temperature carefully lift up the puff pastry using the baking paper and place it onto the hot tray.
Blind bake the puff pastry sheet until golden brown all over. About 20 minutes. Remove from the oven and allow to cool. Press down the centre of the pastry so you are left with a puffy border.
Whisk the ricotta, lemon zest, lemon juice and oregano together. Season well with salt and pepper.
Spread the mixture into the centre of the tart. Top with portabello mushrooms and season them well.
Toss the baby greens in a little olive oil & lemon juice and olive oil and scatter them over the tart before serving.