Lay the cauliflower on a parchment-lined baking sheet. Spray lightly with cooking spray. Roast for 15-20 minutes until cauliflower is tender, but not mushy.
Transfer cauliflower to the bowl of a food processor and add the yogurt and butter. Season with salt and pepper, and process until smooth and creamy.
Heat a skillet over medium-high heat and add the olive oil. Add the veggie crumbles and cook until browned. Add the onion, carrots, and peas and stir until well combined. Combine the veggie broth with the cornstarch and stir until dissolved. Add this and the ketchup to the veggie mixture and stir.
Spread the vegetable mixture in an 8x8 casserole dish or divide into individual ramekins. Top with the pureed cauliflower and spread it on top.
Bake for 25 minutes until heated throughout. To brown the top, turn the broiler on high and broil until the top is browned to your liking. Alternatively, if you have a kitchen torch, you can use that to brown the top. Serve hot.