Preheat your oven to 400 F
In a large oven-safe sauté pan (I’m using a Lodge cast iron pan), heat the EVOO on medium heat. Add the chopped rosemary to infuse the oil. Let it toast for only a few seconds, until the air smells devine. Then add onion and garlic, and sauté until soft.
Add the diced carrots and celery. Sauté for a few minutes, then add lamb. Cook for 8-10 minutes or until the meat is browned, using a spatula to break up the meat in the pan (see video).
Add the broth, tomato paste, rosemary, thyme, salt and pepper. Stir well together to incorporate the tomato paste. Add frozen peas. Simmer for about 5 minutes, until the sauce is thicker, like ragù.
Turn off the stove and use a spatula to flatten the meat mixture into an even layer.
Spoon the mashed cauliflower on top of the layer and spread it evenly, making sure to cover the edges.
Bake the Shepherd's Pie for 30 minutes, until golden around the edges.
Put it under the broiler for 2 minutes for a deeper golden brown crust around the edges. I highly recommend this step!
Remove the Sherperd’s Pie from the oven and let it rest 3-5 minutes before digging in.