Heat a large pot over medium-low heat and add the butter. Add in the shallots, garlic, salt, pepper, and basil. Stir to combine. Cook for 5 to 6 minutes, until the shallots soften.
Stir in the curry paste, brown sugar and tomato paste. Cook for 5 more minutes, stirring often, until the tomato and curry pastes are a deeper color.
Stir in the stock and tomatoes, making sure to scrap the bottom of the bowl. Stir in the coconut milk until combined. Bring the mixture to a boil, then reduce it to a simmer. Add the tortellini right before serving and cook until tender, about 5 minutes.
Serve the soup immediately with fresh basil for garnish and the brown butter garlic toasts.