To add milk to hot soup you first temper the milk by gently warming the milk using a small amount of the hot soup.
Take a small amount of the hot soup and add the small amount of hot soup to the milk before adding the milk to the hot soup.
Tempering the milk before adding it to the hot soup, equalizes the temperatures of the milk and hot soup and prevents the milk from coagulating in the hot soup.
Before adding the milk to the hot soup, take the pot of hot soup off the source of heat.
Then in a separate bowl or container, pour a small amount of the hot soup into the milk.
Whisk the milk and soup together until the milk has heated to a lukewarm temperature.
Then slowly and gradually add the tempered milk back into the main pot of soup and stir constantly.
Next simmer the soup but don't boil it.
Return the soup to a simmer, but don't let it boil as it will cause the milk in the soup to curdle.
You can also add some more milk as needed to reach the desired consistency.
Using a higher fat milk can also help lessen the chance of curdling of the milk in the hot soup and avoid acidic ingredients like tomatoes which is more likely to curdle milk.
And be sure to stir frequently as stirring helps prevent the milk from settling and curdling.
If you want a creamier texture you can also use tempered cream instead of milk.
To temper heavy cream you gradually increase the temperature of the cream by adding a small amount of some hot liquid like sauce or broth to the cream.
The process of tempering the heavy cream prevents the cream from separating or curdling when it's added to a hot dish or hot food.
When tempering the heavy cream, start by turning down the heat on the hot liquid that you're adding the heavy cream to.
Then add a small amount of the hot liquid to the heavy cream by taking a tablespoon or two of the hot liquid and then add it to the heavy cream in a separate bowl.
Next mix the heavy cream and hot liquid thoroughly by stirring it well.
Then repeat the process and gradually add some more hot liquid to the heavy cream, while stirring continuously, until the heavy cream has warmed up and is mixed well with the hot liquid.
You can also take about half a cup of the hot broth and then slowly add the heavy cream to the hot broth while stirring constantly.
If you want to quickly bring heavy cream to room temperature you can microwave the heavy cream in a microwave safe bowl, with 10 second intervals, stirring between each pulse.
You can also put the heavy cream in a bowl and place the bowl in a larger bowl of warm water for 5 to 10 minutes.
Or you can simply leave the heavy cream on the counter covered for around an hour to bring the heavy cream to room temperature.