Preheat the oven to 425° and position a rack in the upper third. Turn the chicken legs skin side down on a work surface and cut halfway through the joint. Season the chicken generously with salt and pepper.
In each of 2 large nonstick skillets, heat half of the butter and oil. Add the chicken, skin side up, and cook over high heat until browned, 5 minutes. Turn and cook the chicken for 1 minute. Pile all of the chicken into one skillet.
In the other skillet, cook the leeks over high heat until just beginning to soften, about 2 minutes. Add the broth and vinegar and bring to a boil. Season with salt and pepper. Pour the mixture into a medium roasting pan.
Set the chicken on the leeks, skin side up; roast for about 25 minutes, until it is cooked through. Turn on the broiler and broil for about 2 minutes, until the skin is golden and crisp. Transfer the chicken to a platter.
Place the roasting pan over a burner and boil over high heat until the liquid is reduced by half, about 5 minutes. Add the peas, herbs and crème fraîche and simmer until the sauce is hot and slightly thickened, about 2 minutes. Season with salt and pepper. Pour the sauce over the chicken and serve.