Place an oven rack in the lowest part of the oven. Preheat the oven to 450 degrees F. Line a large, sided baking sheet with parchment paper. Generously season the chicken with salt and pepper and set, placing the thighs skin-side down, on the baking sheet. Roast 10 minutes. Remove the chicken from the oven and turn the pieces over. Roast 15 minutes more. Divide the chicken between 2, 9- x 13-inch baking dishes. Reduce the oven temperature to 325 degrees F.
Heal the oil in a wide pot set over medium to medium-high heat. Add the leeks, mushrooms, carrot and celery and cook until tender, about 5 minutes. Mix in the garlic and flour and cook, stirring, until the flour is well incorporated. Stirring steadily, slowly mix in the stock, and then the wine. Add the sage and bring to a simmer; season with salt and pepper. Divide and pour the mixture over the chicken. Cover and bake the chicken 60 to 70 minutes, or until the chicken is very tender. Sprinkle the chicken with the peas, heat them through a few minutes and serve.