Combine lemon juice, corn starch, cumin, and 1/2 tsp each of turmeric, garam masala, and salt in a large bowl. Set aside.
Heat 1 Tbsp of the oil in a large skillet over medium-high. Cook cauliflower, tossing occasionally, until browned in places and beginning to soften, 7 to 8 minutes. Reduce heat to medium, and transfer cauliflower to bowl with corn starch mixture and toss to coat. Return seasoned cauliflower to skillet and cook another 7 to 8 minutes, until charred in spots and crisp-tender. Transfer cauliflower to a bowl.
Add remaining 1 Tbsp oil and butter to skillet. Add onions and cook until translucent, about 8 minutes. Stir in garlic, ginger, and tomato paste; cook 2 minutes, until fragrant. Add remaining 1/2 tsp turmeric, 2 1/2 tsp garam masala, and 1 tsp salt, along with paprika, cinnamon, and cayenne; cook 1 minute. Add tomato sauce and broth; bring mixture to a boil. Reduce to a simmer, stir in cream, and return cauliflower to skillet. Simmer mixture uncovered until sauce thickens, about 15 minutes. Garnish with cilantro and serve over rice with a dollop of yogurt.