Asparagus tart with sour cream pastry

Pastry

• 250 g unsalted butter, cut into cubes, plus extra for greasing
• 325 g plain flour, plus extra for dusting
• 1 pinch fine salt
• 160 g sour cream

Filling

• 500 g water
• 200 g asparagus, woody ends trimmed
• cold water, for soaking
• 100 g cheddar cheese, cut into cubes (2 cm)
• ½ bunch fresh chives, cut into small pieces (2 mm)
• 1 lemon, zest only, finely grated, no white pith
• 100 g ricotta
• 3 eggs
• 100 g sour cream
• ½ tsp sea salt flakes
• ½ tsp ground black pepper
• 6 spring onions/shallots, cut into pieces same length as asparagus, then cut into halves