Preheat oven to 325 degrees Fahrenheit
For mashed cauliflower: Fill a large pot of water and add chopped cauliflower. Bring water to a boil and cook cauliflower for 10-15 minutes or until a fork pierces cauliflower easily.
Drain cauliflower from water and transfer to a blender, food processor or vitamix. Add butter, milk, garlic cloves, thyme sprigs, kosher salt and pepper to cauliflower and puree until smooth. Set aside for topping Shepherds pie…
For shepherds pie: In a dutch oven or heavy bottom pot heat olive oil on medium heat for 1-2 minutes. Add in onions, shallots, minced garlic, carrots and mushrooms and saute for 5-10 minutes or until onions are translucent.
Sprinkle vegetables with flour and add tomato paste and mix well. Add in red wine and cook for an additional 2-3 minutes or until most of the liquid has been absorbed by the vegetables.
Add in the thyme sprigs, vegetable broth and peas and stir consistently for 5-10 minutes on medium low heat until mixture has thickened. Stir in the cooked green lentils and transfer mixture to a casserole dish (if not using a dutch oven).
Top mixture evenly with mashed cauliflower and sprinkle with a later of grated parm. Bake for 30-35 minutes or until pie is slightly golden.