Preheat the Oven
Begin by heating the oven to 425°F. Arrange the chicken thighs and red onion wedges on a rimmed sheet pan. Season with 1½ teaspoons of salt and 1 teaspoon of pepper. Add the tarragon and 1½ tablespoons of olive oil, tossing everything together. Allow the mixture to rest at room temperature as you prepare the remaining ingredients.
Simmer the Vinegar Solution
In a small saucepan, combine the rice vinegar, 2 tablespoons of sugar, the bay leaf, and the remaining teaspoon of salt. Stir frequently over medium-high heat until the sugar dissolves and the mixture comes to a simmer.
Prepare the Cucumbers
Place the thinly sliced cucumbers in a medium-sized bowl. Pour 2 tablespoons of the simmered vinegar solution over them and toss gently to coat. Set the cucumbers aside for now.
Cook the Cherries
Return the saucepan to medium-low heat, adding the final tablespoon of sugar. Stir until the sugar dissolves, then add the sour cherries, cooking them for 3 to 4 minutes until they begin to break down slightly. Taste the mixture, adjusting the sugar as needed to balance the tartness of the cherries.
Roast the Chicken
Spoon the cherry mixture and its juices over the chicken and onions. It is acceptable for some cherries to rest on top of the chicken. Roast everything for 30 to 35 minutes, until the chicken is fully cooked and the cherries and onions are lightly browned. Stir the ingredients, then spoon the pan juices over the chicken for added flavor.
Prepare the Cucumber Yogurt
While the chicken roasts, mix the yogurt and the remaining 1 tablespoon of olive oil with the cucumbers. Season with salt and pepper, then refrigerate until ready to serve. Stir before serving and adjust seasoning if necessary.
Serve
Once the chicken and cherries are ready, top them with a generous dollop of cucumber yogurt, a sprinkle of extra tarragon, and a drizzle of olive oil. Optionally, finish with a light dusting of flaky sea salt or more kosher salt to taste.