1. Preheat oven to 400°F. Blend onion, tomato sauce, jalepeño, ginger, garlic, garlic, and cumin in blender until paste forms. Heat 2 Tbs. oil in skillet over medium-high heat. Add onion paste, and cook 3 minutes, or until mixture comes together in dense mass. Stir in coriander, turmeric, cayenne, amchoor, and garam masala; cook 2 minutes. Stir in 1/2 cup water and grated potato.
2. Steam whole cauliflower head 5 to 7 minutes. Cool. Press onion mixture all over cauliflower. Set in baking dish.
3. Toss sliced potatoes with remaining 1 Tbs. oil on baking sheet. Transfer both pans to oven, and bake 25 minutes, turning potatoes once. Serve cauliflower with sliced potatoes.