The secret that the chefs hide! Here's how to make the most tender meat!

The secret that the chefs hide! Here's how to make the most tender meat!
Components:
beef: 1 kg
milk: 500 ml
RELAX: 20 MIN
olive oil: 50 ml
butter: 80 g
onions: 2 pcs
mushrooms: 100 g
garlic: 10 g
salt: 10 g
black pepper: 5 g
sweet pepper: 5 g
mustard: 30 g
soy sauce: 30 ml
water: 500 ml
starch: 30 g
water: 30 ml
spring onion: 10 g
parsley: 10 g