If not already on, preheat the oven to 300˚F (150˚C).
Spread 1 tablespoon of softened butter on one side of each slice of bread. With the buttered side facing out, layer on the sliced cheddar and Havarti and top each sandwich with a second piece of buttered bread.
In a large pan over medium heat, toast each sandwich, buttered side down, until golden brown, about 2 minutes. Flip and repeat on the other side, then transfer to a baking sheet.
Bake the grilled cheeses for 5-10 minutes, until the cheese is gooey and fully melted.
Cut in half on the diagonal and serve warm with roasted tomato soup.