My hubby makes his own BM.
He makes a big bowl full that sits in the fridge and keeps us going for most of the week. it doesn't need to be fresh everyday, it improves with age
Into a 2 liter bowl grate 1 apple (unpeeled) we prefer Braeburn.
Then add 2 large handfuls of Migros Bio
Reddy Birchermuesli Berries mix and stir into the apple.
Next throw in some sultanas - only if you like them
Next add 1 large tub 500g of Migros mixed berry or other flavour - stir again.
Then put a mug full of [still] frozen mixed forest berries - stir again and then add enough milk to make it quite a sloppy mixture - it will thicken a lot overnight. This part needs and a bit of experimentation and tinkering to personal taste.
Cover and put in the fridge overnight.
The next day to will be very thick or solid and this is the time to add more milk/yogurt/orange juice (if you like it less sweet) to loosen it up, give it a good stir.
Scoop into a bowl and enjoy!!
The remainder will last 4-5 days in the fridge, but it wont thicken any more.