1. Prepare the Lamb Marinade:

In a small bowl, combine the olive oil, rosemary, thyme, salt and pepper

Rub the marinade generously over the rack of American lamb, ensuring the meat is well coated. 

Cover and let the lamb marinate at room temperature for 30 minutes, or refrigerate for up to 4 hours (bring to room temperature before roasting if refrigerated).

2. Roast the Lamb

Preheat the oven to 425°F (220°C).

Place an oven-safe skillet over medium high heat.  Once hot, sear the lamb 

fat side down for 3-4 minutes until lightly browned.  Turn the lamb over in the skillet.

Transfer the skillet into the preheated oven. Roast for 15-20 minutes for medium-rare (or until the internal temperature reaches 130°F/54°C), or longer if you prefer the lamb more well done.

Remove from the oven and let the lamb rest, for about 15 minutes before slicing.

5. Prepare the Leek and Mushroom Sauté:

While the lamb is roasting, heat butter and olive oil in a large skillet over medium high heat.

Add the sliced mushrooms and cook for 5-7 minutes, stirring occasionally, until the mushrooms release their moisture and begin to brown.

Reduce heat to medium and stir in the leeks, minced garlic and thyme, cooking for an additional 5-7 minutes.

Deglaze the pan with the white wine, scraping up any browned bits from the bottom of the skillet, let it cook till it evaporates.  

Stir in the dijon mustard and heavy cream, season with salt and freshly ground black pepper, and cook for 2 more minutes until everything is well combined and glossy.

6. Serve:

Slice the rested rack of lamb into individual chops.

Serve the lamb chops alongside the leek and mushroom sauté, garnishing with fresh parsley if desired.

Tips:

For perfectly tender lamb, use a meat thermometer to check the internal temperature: 125°F (52°C) for rare, 130°F (54°C) for medium-rare, and 140°F (60°C) for medium.