Sift the flour and a pinch of salt into a large bowl; rub in the butter until the mixture resembles fine breadcrumbs.
Dissolve yeast in the warm water; add to flour-butter-salt mixture. Mix egg and water; sprinkle onto the flour mixture. Work ingredients together to make a dough.
Turn the dough onto a lightly floured work surface; knead briefly until smooth. Place in bowl, cover with cling wrap and leave at room temperature for 45 minutes.
Place the garlic in the bottom of a steamer filled with about 5 cm of hot water. Place asparagus stalks in the top part of the steamer and steam for 1 minute over a moderate heat. Add asparagus tips; steam a further 1–1½ minutes or until just starting to soften. (Thicker spears may need a little longer.) Remove garlic and asparagus from steamer; set aside.
Preheat oven to 200°C and put a baking tray in to heat. Roll out the dough thinly (you don't have to knead it again) on a lightly floured work surface and line the flan tin.
Scatter pecorino cheese over the base of the pastry case, then arrange the asparagus and prosciutto on top, filling the gaps with peas.
Squeeze the softened garlic from the skins and mash to a paste. Whisk in the eggs and egg yolk, and season with salt and pepper. Heat milk to boiling point; gradually add it to the egg mixture, whisking constantly to combine.
Place the tart on the hot baking tray. Pour egg mixture into pastry case.
Bake 5 minutes, then reduce oven temperature to 180°C. Bake for a further 25–30 minutes or until filling is lightly set and the pastry is golden brown.
Leave tart in tin for 5 minutes. Serve hot or warm.