1 Rinse rhubarb, drain, trim and cut into 5 mm (approximately 1/4-inch) thick slices. Mix rhubarb in a bowl with 30 grams (approximately 2 tablespoons) sugar and let stand for 10 minutes.
2 Meanwhile rinse lime in hot water, dry, grate peel finely and mix zest with rhubarb.
3 Cut vanilla bean lengthwise, scrape out seeds and bring to a boil in a small pot with milk and butter. Sprinkle spelt semolina into pot and bring to a boil, stirring constantly with a wooden spoon.
4 Separate eggs. In a high vessel, beat egg whites with a pinch of salt until stiff with a hand mixer. Gradually add 50 grams (approximately 1/4 cup) sugar and continue to beat until sugar dissolves.
5 Mix quark, egg yolks and rhubarb into warm spelt batter then fold in egg whites.
6 Line a 12-cup muffin tin with paper baking cups. Pour batter into cups and sprinkle with remaining sugar.
7 Bake in preheated oven at 200°C (fan: 180°C, gas mark 3)(approximately 400°F/convection 350°F) on middle oven rack for about 25 minutes.
8 Remove tin from oven and let cool for 5 minutes. Then remove muffins from tin and let cool completely on a wire rack for about 1 hour before serving.