Heat oven to 200C/180C fan/gas 6. Unroll the pastry but leave it on the parchment, then transfer it to a baking tray. Brush egg yolk all over. Trim the edges with a large, sharp knife – this will give you a neater finish but isn’t essential. Cut the pastry in half lengthways, and move the pieces apart so they have a little space to puff up as they cook. Score a border about 1cm from the edge of the pastry, all the way around, then cut slashes across the centre to prevent it from puffing too much. Be careful not to score the border.
Toss the carrots and radishes in a little oil and a sprinkling of salt in a small roasting tin. Put the pastry on the top shelf of the oven and the veg on the middle one, and cook for 25 mins, or until the pastry is golden and has a crisp base, and the veg are starting to soften. Leave to cool while you make the lemony feta.
Crumble the feta into a bowl, add the mascarpone, lemon zest and milk and blend using an electric hand whisk until smooth. Chill until needed. To make the salsa verde, put all the ingredients in a small hand processor and whizz to a pesto consistency. This will keep in the fridge for a few days.
Bring a pan of water to the boil, and have a bowl to hand filled with cold water and some ice. Add the asparagus to the boiling water and cook for 1-2 mins until bright green but still crunchy. Use a slotted spoon to scoop out and plunge straight into the iced water. Do the same with the sugar snaps, and finally the peas, draining these in a sieve, then cool in the ice water with the other veg.
Spread the whipped feta over the bases, top with the veg (you can mix them up or keep the roasted veg on one tart and the green veg on the other, if you like), then drizzle with salsa verde and a little oil. Best served within an hour or two, but leftovers will keep for a day in the fridge.