Place tomatoes cut-side down on a baking sheet lined with foil for easy cleanup and brush skin with 1 tablespoon of the grapeseed oil, reserving the rest of the oil. Roast tomatoes until charred, about 15 minutes, remove from oven and place in plastic bags to sweat. This will help make it easier to remove the skin.
Heat chicken stock in a medium saucepot over medium heat. Peel shrimp and add shells to the pot, reserving the shrimp. Bring the pot of stock and shells to a boil, skimming any impurities which rise to the surface.
In a large stockpot, heat the remaining grapeseed oil over medium heat and melt the butter in it, and saute onions and garlic until onions turn translucent. Add the mushrooms and cook until they begin to soften and give up their juices. Strain the chicken stock into the stockpot with the onions and mushrooms and discard the shrimp shells. Remove skin from the roasted tomatoes and add them to the pot. Add the white rice and all-purpose seasoning, bring to a gentle boil, reduce heat to low, cover and let simmer 20 minutes.
Chop most of the shrimp, small dice, reserving about 8 whole shrimp. Stir chopped shrimp into pot and add whole shrimp last, without stirring in the whole shrimp. Cover pot and let cook for 5 more minutes. Remove from heat and scoop out whole shrimp and transfer to a utility plate and cover to keep warm. Stir heavy cream into pot and puree with an immersion blender and season with salt and pepper. Spoon into soup bowls and garnish with whole shrimp topped with cilantro and mint. Serve with croutons.