gotować się na wolnym ogniu in inglese:

I simmer meat.

Please simmer the beans for a while over a low heat.

Simmer the butter and garlic in a small saucepan.

Next, peel two onions and slice them into strips. Throw it to the pot and simmer for about twelve minutes.

The soup was simmering on the stove.

Leave the ​vegetables to simmer for a few ​minutes.

Bring a large saucepan of lightly salted water to the boil over medium hight-heat. Add cabbage and simmer for 5 minutes.

to boil gently, or to cook something slowly by boiling it gently: American EnglishBring the soup to the boil and allow it to simmer gently for about half an hour.

Such elements can run at as low a cooking-heat level as wanted for gentle simmering and suchlike

To simmer something means to cook it just below the boiling point.

leave to simmer

Done! Now I just have to set it to simmer so it doesn't boil before everybody comes.

Be sure to simmer on a low heat so it doesn't boil.