Thinly slice 1 medium yellow onion and finely chop 4 garlic cloves.

Pat 1 pound thin-cut boneless, skinless chicken breasts dry with paper towels. Season all over with 1 teaspoon kosher salt and ½ teaspoon black pepper. Place ¼ cup all-purpose flour on a plate. Working with one piece of chicken at a time, press into the flour until evenly coated all over. Place them on a plate on a single layer.

Heat 2 tablespoons olive oil in a 10-inch skillet over medium heat until shimmering. Add half of the chicken and cook until lightly browned, about 1 ½ minutes per side (the chicken will not be cooked through). Transfer to a plate and repeat cooking the remaining chicken.

Meanwhile, place ½ cup low-sodium chicken broth, ½ cup heavy cream, ¼ cup sour cream, 1 tablespoon soy sauce, 1 teaspoon apple cider vinegar, and ¼ teaspoon garlic powder in a medium bowl or liquid measuring cup and whisk to combine.

When the chicken is ready, add the onion, 1 tablespoon unsalted butter, and a pinch of kosher salt to the now-empty pan. Reduce the heat to medium-low and cook, scraping up any browned bits from the bottom of the pan with a wooden spoon, until the onion is starting to brown, 5 minutes. Add the garlic and ¼ teaspoon red pepper flakes, and cook until fragrant, about 30 seconds.

Add the chicken broth mixture and stir to combine. Cook until the sauce starts to thicken, 3 to 4 minutes. Gradually add 5 ounces baby spinach and cook, stirring often, until the spinach is wilted.

Return the chicken and any accumulated juices on the plate to the skillet. Increase the heat to medium high and cook until the chicken is cooked through and coated with sauce, 4 to 5 minutes. Serve over cooked pasta or rice.