You add the sour cream last to avoid curdling. However, you have other options as well.
The protein in milk products is what curdles. The higher the fat content, the longer it can be heated without curdling, until it hits a point in the high 30s (I think it’s 38%? I can look that up later) where it can be boiled without curdling. While American sour cream is below this point, creme fraiche and European sour creams are typically above this point.
Which is my roundabout way of saying that you can use a higher-fat dairy option and worry less about when you add it. Or, I guess, add it last. That’s good, too.
From a box sold in Louisville, Kentucky.
1 stick butter
1 can mushrooms
3 onions (green) cut up or 1 old
1 teaspoon ginger
Cook [mushrooms, presumably?] until tender in [butter, presumably?] in skillet. Add 1 cup sliced cooked chicken and heat. Add 1/2 cup sour cream. Serve on rice. (Add sour cream last.)