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 very large, deep skillet. Heat over medium-high heat, 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 the 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 1 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 cool down a bit. 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
Make sure not to overcook the chicken pieces. You want them juicy and succulent. In the first stage of cooking them, cook just until they're no longer raw. They will cook some more when you add them back to the skillet.The leftovers keep fairly well in the fridge, in a sealed container, for about 3 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.