Wash and peel the asparagus and cut off the woody ends. Peel and wash the carrots and cut them in half lengthwise. Cut both diagonally into pieces. Clean, wash and halve the sugar snap peas. Peel and chop the onion.
Let the puff pastry rest for about 10 minutes at room temperature. Cover the asparagus and cook in boiling salted water with 1 teaspoon sugar for 5-6 minutes. Add sugar snap peas and cook for 1 minute. Drain, quench and allow to drain.
Heat the butter, fry the carrots and onion for 3-4 minutes. Season with salt, pepper and 1 pinch of sugar. Mix the vegetables, let them cool down a bit.
Stir the sour cream and eggs together. Season with salt and pepper. Grate the Gouda roughly. Dice mozzarella.
Unroll the dough together with the baking parchment and place it with the paper in a rectangular tart or baking tin (approx. 20 x 28 cm). Roll up protruding edges slightly. Fill in vegetables. Pour over the mixture.
Spread both cheeses on the tart.
Bake in the preheated oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer) for approx. 35 minutes. Take out the asparagus tart and let it rest for about 5 minutes
Drink tip: dry white wine, e.g. a Silvaner