Measure out the sour cream and remove it from the fridge to allow it to warm up a little. Season the chicken with half the kosher salt and black pepper.
Add 1 tablespoon of olive oil to a large (12-inch), deep skillet. Heat over medium-high heat for about 2 minutes. Add the chicken. Cook, stirring often, until no longer raw, 5-7 minutes. Remove to a plate.
Add the remaining olive oil to the skillet. Add the onions and mushrooms and sprinkle them with the remaining kosher salt and black pepper. If the skillet seems too dry, add a splash of water. Cook, stirring often, until the vegetables are tender and browned, about 7 minutes.
Stir in the garlic and cook for one more minute. Add the chicken back to the skillet and stir-fry until all chicken pieces are cooked through, about 2 more minutes.
Reduce the heat to low and allow the skillet a few seconds to slightly cool. Add the sour cream, mixing it in with a spatula or a wooden spoon. Don’t allow the sour cream to simmer or boil, to avoid curdling. Serve immediately.
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Notes
If using any salt other than Diamond Crystal Kosher Salt, including Morton kosher salt, use half the amount listed.
This recipe is not very saucy. If you want more sauce, add ½ cup of chicken stock after adding back and sauteing the chicken, and before adding the sour cream.
You can replace the chicken breast with boneless chicken thighs.
Don't overcook the chicken. You want it juicy and succulent. In the first stage of cooking, cook it just until it's no longer raw. Remember that it will cook some more when you add it back to the skillet after cooking the onions and mushrooms.
You can use rotisserie chicken. Cube the chicken breasts (remove the skin first), cook the onions and mushrooms, and add the chicken pieces, skipping the first stage of cooking the chicken. The only caveat: Rotisserie chicken breasts are often quite dry.
You can keep the leftovers in a sealed container in the fridge for about three days. However, they're not as good as the freshly made dish. Chicken tends to dry out when reheated. I recommend reheating the leftovers very gently to avoid drying the chicken and curdling the sour cream. Use your microwave at 50% power, and reheat them covered.