• 2 tbsp. heavy cream (or more to taste)
Put the butter, 3 tablespoons of oil, onions, and 1 teaspoon of salt into a large sauté pan on medium heat and cook for 18 to 20 minutes, stirring often, until soft and deeply golden brown. Lower the heat if they get too brown.
Transfer two-thirds of the fried onions to a bowl and stir in the remaining 3 tablespoons of olive oil.
Return the pan with the remaining onions to medium heat, add the garlic, and fry for 2 minutes. Add the tomatoes, tomato paste, basil, habañero (if using), and 2 teaspoons of salt and fry for 7 minutes, stirring often. Increase the heat to medium-high, add the stock, 1 1/4 cups of water, and a good grind of pepper and bring to a simmer. Lower the heat to medium and cook for 12 minutes. Remove the habañero, if using, and squeeze to discard any liquid. Finely chop the habañero and stir into the bowl with the reserved fried onions.
Let the soup cool for 5 to 10 minutes, so it's not dangerously hot, then transfer to a blender and blend until completely smooth.