Prepare the fruit: wash and hull the Strawberries, halve or quarter larger berries; leave the Raspberries and Red Currants whole and pit the Sour Cherries if they are fresh. Place all the fruit in a bowl while you assemble the poaching liquid.
Combine the fruit with the Red Fruit Juice, the Granulated Sugar, and a splash of Lemon Juice in a medium saucepan. Stir gently and let the mixture sit briefly so the sugar begins to draw out the juices.
Bring the pan to a gentle simmer over medium heat, stirring occasionally until the fruits are softened and aromatic (a few minutes). Taste and add more sugar or lemon juice if you prefer it sweeter or brighter.
Make a slurry by whisking the Cornstarch with a little cold red fruit juice or water until smooth, then pour the slurry into the simmering fruit while stirring constantly. Continue cooking for 1–2 minutes until the liquid thickens into a glossy compote; remove from the heat and let cool, then chill in the fridge so it sets.
While the grütze chills, prepare a vanilla custard: split the Vanilla Bean and scrape out the seeds. Warm the Whole Milk and Heavy Cream with the vanilla seeds and pod in a saucepan until just below boiling, then remove from the heat and let infuse for a few minutes.
Whisk the Egg Yolks with a few spoonfuls of the remaining Granulated Sugar and a pinch of Salt until the mixture is pale. Temper the yolks by slowly whisking in a little of the hot milk mixture, then pour everything back into the pan.
Return the custard to low heat and cook, stirring constantly, until it thickens enough to coat the back of a spoon (do not boil). Strain the custard through a sieve to remove the vanilla pod and any cooked bits, then cool slightly—serve warm or chilled.
To serve, spoon the chilled Rote Grütze into bowls and ladle over the vanilla custard or a dollop of softly whipped cream. Garnish with a few whole berries if desired and enjoy immediately.
