Mix butter, flour, egg, nuts, salt and 1-2 tablespoons of cold water first with the dough hooks of the hand mixer, then with your hands to a smooth dough. Press dough flat, wrap in foil and refrigerate for approx. 45 minutes.
Leek onions wash, clean and depending upon thickness lengthwise halve or quarter. Wash, peel and clean the black salsifies. Cut black salsifies lengthwise into slices about 2 mm thick.
Heat the oil in a large frying pan. Sauté the black salsifies in it for 4-5 minutes, leaving only a little colour. Add spring onions and fry for about 1 minute, season with salt and pepper and remove.
Roll out the dough oval (approx. 40 x 25 cm) on a floured work surface and place on a baking tray lined with baking paper. Season the sour cream with salt and pepper and spread it on the base, leaving a rim of approx. 3 cm all around.
Spread the spring onions and black salsify on the cream. Whip the edges lightly over the filling.
Bake the tart on the lower shelf in a preheated oven (electric oven: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for 35-45 minutes. Cut the bacon into pieces and spread it on the tart about 10 minutes before the end of the baking time, bake with it.
Sprinkle the pine nuts over the top about 5 minutes before the end of the cooking time and bake with them. Remove the tart from the oven and serve immediately.