Preheat the oven to 350°F (175°C) and lightly grease a 9-by-13-inch baking dish.
Separate the sliced onion into rings and spread them evenly across the bottom of the baking dish.
Pat the chicken thighs dry and arrange them over the onions in a single layer, skin-side up.
Pour the Russian dressing evenly over the chicken, using a spoon to spread it over each thigh if necessary.
Bake uncovered for 45 to 55 minutes, or until the chicken is cooked through and an instant-read thermometer inserted into the thickest portion registers at least 165°F (74°C).
Remove the baking dish from the oven and let the chicken rest for about 5 minutes.
Serve the chicken with the tender onions and spoon the pan sauce over the top.
Notes
For even cooking, choose chicken thighs that are fairly similar in size and avoid overlapping them in the baking dish.
Cooking time can vary depending on the thickness of the chicken, so always check doneness with a meat thermometer rather than relying only on the clock.
Don't discard the onions or sauce left in the pan. Spoon them over mashed potatoes, rice, or egg noodles when serving.