For the stuffing: Spread corn and onion on a baking pan, toss with grapeseed oil to coat, and sprinkle with turmeric, salt and pepper. Roast until corn is tender, about 25 to 35 minutes. (Leave oven on to warm tomatoes after stuffing them.)
For the puree: While corn and onion are in oven, combine Brussels sprouts, cauliflower, and stock in a small but deep saucepot (you will be using an immersion blender). Cook until vegetables are soft and liquid is reduced by half. Puree using an immersion blender. Add cubed butter and season with salt and pepper, to taste. Set aside and keep warm.
For the tomatoes: Blanch tomatoes for 1 or 2 minutes in salted boiling water and slip off skins. Cut in half and remove seeds. Place on baking sheet and stuff with corn/onion mixture. Bake in oven until heated through, about 5 to 7 minutes.
Stir puree of Brussels sprouts and spoon onto plate. Place 2 halves of a stuffed tomato in center.