Heat a large oven-safe frying pan on medium-high heat, and add the olive oil and butter. Season both sides of the chicken generously with salt and pepper. Once hot, lay the chicken thighs skin side down, ensuring not to crowd the pan. Cook for 5 minutes on one side, or until golden brown, then flip and cook for an additional 3 minutes. Transfer the browned chicken thighs to a plate.
Reduce the heat to medium and add the leeks, garlic, and thyme. Cook for 3-4 minutes or until the leeks are softened and fragrant.
Pour in the white wine, and stir, scraping the bottom of the pan to release any bits that might have stuck to the bottom. Cook for 2 minutes to slightly reduce.
Then add the chicken broth and whipping cream. Increase the heat to medium-high, and cook for 2-3 minutes or until slightly thickened. Then stir in the two mustards.
Lay the chicken thighs, skin side up, on the sauce. Transfer to the oven, and bake for 20-25 minutes, or until the internal temperature of the chicken is 165°F.