Add a steamer basket to a large pot along with about 1 inch of water at the bottom of the pan. Heat the water to boiling, and add the chopped cauliflower and potatoes to the steamer basket. Lower to medium heat, and add a lid to the pot. Steam the vegetables for 15-20 minutes, until fall-apart tender when pierced with a fork. If your vegetables still have a crunch to them, the mash will turn out lumpy instead of really creamy.
Scoop steamed vegetables into a food processor, along with salt and pepper. Process until the vegetables are very creamy. *Tip: Make sure the steamed vegetables are not water logged. Drain excess moisture before adding to the food processor. Set mash aside.
Preheat the oven to 400º F. In a large skillet, heat up 1 tablespoon olive oil on medium high heat. Add onion, garlic, rosemary, salt and pepper, and saute for 7-10 minutes, until just beginning to brown.
Add the ground beef, and brown, while stirring to break up the meat, about 10 minutes.
In a small bowl, add beef broth, tomato paste, coconut aminos, and arrowroot powder, and whisk together until combined well.
Add the beef broth mixture over the ground beef, and bring to a low simmer over medium heat. Whisk so the arrowroot thickens without clumping.
Add the carrots and snap peas, and stir to combine, and then remove from heat.
Add the ground beef and vegetable mixture to the bottom of a 9x9 or 8x10 casserole pan.
Top the mixture with the cauliflower-potato mixture, spooning out big scoops and placing them on the corners of the casserole. Use a spoon to smooth the potatoes into an even layer. Top with more rosemary and black pepper, if desired.
Bake in the Sharp Superheated Steam Countertop Oven on 400º F on the Bake/Reheat function for 14 minutes, or until the top has begun to brown. If you've refrigerated the casserole prior to baking, it will take an extra 5-8 minutes to fully heat through.
If using a conventional oven, preheat the oven to 400º F, and bake the casserole for 22 minutes, until the top has begun to brown.