• Rinse the leg of lamb and pat dry. 
• Place the onion, potatoes and carrots (if using), garlic, and remaining sprigs of herbs in the bottom of the slow cooker, creating a shelf for the lamb leg to sit on.
• In a small bowl mix the salt, pepper, lemon zest, garlic powder, thyme, and rosemary together.
• Rub olive oil over the lamb and then coat with the herb mixture.
• Preheat a large skillet with olive oil to medium-heat. Once it just starts to smoke. Place the lamb in the skillet and sear all the sides until you get a nice golden color.
• Transfer the lamb leg (and any juices from the skillet) to the slow cooker and place on top of the vegetables and herbs. (Tip: If you have any flavorful stuck bits on the bottom of the skillet, deglaze the pan by adding a couple of tablespoons of wine, loosen with a spatula, and transfer to the crockpot or save for your gravy.)
• Cover and cook on low for 4.5 to 8 hours or until roast reaches your desired temperature and doneness using a meat thermometer (see temperature chart below). Be careful not to overcook your roast. Check the temperature periodically.
• Remove from slow cooker and tent loosely with foil. Allow to rest for 5 to 10 minutes before slicing. See notes below for making gravy.

To make gravy

• Strain the solids from the liquid after cooking. Transfer the juices to a skillet. Make a slurry with 1 to 2 tablespoons of flour whisked into 1 to 2 tablespoons of cold water, beef broth, or lamb broth. Whisk the slurry into the juices in a skillet and simmer over medium to medium-high heat, stirring until thickened. If you need more flour, mix it with a little broth or water first to avoid lumps.