Coat the bottom of a springform pan (26 cm Ø) with oil, with a high rim. Put the rusk into the freezer bag and close the bag. Roll over with a pastry roll until the rusk is finely crumbled.
Melt 150 g butter and mix with the crumbs. Press in the mould to the bottom (rim approx. 2 1⁄2 cm high) and chill for approx. 1 hour.
Wash lemon hot and dab dry. Finely grate half the peel. Squeeze lemon. Melt 50 g butter and let it cool down a little. Separate the eggs. Whisk the egg yolks and 200 g sugar with the whisks of the mixer until thick and creamy.
Gradually add 500 g mascarpone and quark. Stir in melted butter, lemon peel and about half of the juice. Sift the starch on top and fold in. Preheat the oven (electric cooker: 175 °C/circulating air: 150 °C/gas: s.
Beat the egg whites with 1 pinch of salt until stiff, adding 100 g sugar. Fold into the mascarpone mixture and place in the mould. Bake in a hot oven on the lower shelf for about 35 minutes. Remove and cut all around about 2 cm deep between the cake and the edge of the tin.
Let it rest for about 10 minutes, then bake at the same oven temperature for another 20-25 minutes. Let it cool down on a cake rack.
Wash the strawberries, clean them and puree 50 g of them. Halve or quarter the rest depending on size. Mix 250 g mascarpone, 50 g sugar, rest lemon juice and vanilla sugar. Whip cream until almost stiff and fold in.
Carefully remove the cake from the tin and place it on a cake plate. Spread mascarpone cream loosely on the cake. Swirl the strawberry puree into streaks. Add the rest of the berries. Keep cool until serving.