Steam. Add cauliflower florets (24 ounces) to a large saucepan along with 2" of water. Cover and steam until fork tender (about 10 minutes).
Mash. Drain cauliflower, then add them back to the saucepan (if you have an immersion blender, if not, add to a high-speed blender* or food processor). Stir in the heavy cream (1/4 cup), butter (2 ounces), salt (1 teaspoon), pepper (1/4 teaspoon) and egg yolk (1). Blend until cauliflower is smooth and no lumps remain.
While cauliflower is cooking, begin the filling by preheating your oven to 400 F.
Cook meat + veggies. In a large oven-safe pan, heat vegetable oil. Add diced yellow onion (1 medium size) and carrots (2 peeled and diced) and cook until they begin to brown (about 4 minutes). Add minced garlic (2 cloves), and continue to cook until fragrant (about 1-2 minutes). Stir in the ground beef (2 pounds), salt (1 1/2 teaspoon) and pepper (1/2 teaspoon). Break the meat up using the back of a wooden spoon as it cooks. Cook until meat is no longer pink and totally cooked through.
Drain excess fat from the pan -- to do this, I turn the stove off. Then move the meat in the pan to one side, and slightly tip the pan so the fat pools to one side. Then carefully, clean up this excess fat with paper towels. Be sure to leave a few teaspoons of fat in the pan (the amount you remove will vary depending on how high fat the meat you use is!), this is important for creating our sauce.
Reduce. Sprinkle the meat with xanthan gum (1 teaspoon) and stir in tomato paste (2 teaspoons), soy sauce (1 teaspoon), chicken broth (1 cup), finely chopped rosemary (2 teaspoons ) and thyme (1 teaspoon). Bring mixture to a boil, then reduce heat and gently simmer until the sauce thickens. (about 10 - 12 minutes)
Assembly. Pour filling into a baking dish (I used a 9" round cast iron, an 8" square dish will do as well). Scoop mashed cauliflower on top of the filling, making sure to spread evenly and seal the filling completely.
Bake. Place baking dish onto a rimmed baking sheet and into the preheated oven. Bake for 25-30 minutes, you'll know it's done once the top is golden brown and the dish is heated through. Allow pie to cool slightly before serving, and enjoy!