How to Make It
Preheat the oven to 350°. On a rimmed baking sheet, toss the bread cubes with the 2 tablespoons of melted butter and a generous pinch each of salt and pepper. Spread in a single layer and bake for 10 to 12 minutes, until golden brown, rotating the sheet from front to back halfway through. Transfer the croutons to a wire rack.
Meanwhile, in a large saucepan, cook the bacon over moderate heat, stirring occasionally, until golden brown and crisp, about 14 minutes. Using a slotted spoon, transfer the bacon to paper towels. Stir the ground sirloin into the pan, season with a pinch each of salt and pepper and cook over moderate heat, breaking up the meat, until browned, about 5 minutes. Using the slotted spoon, transfer the meat to a bowl. Wipe out the saucepan.
Melt the remaining 2 tablespoons of butter in the saucepan over moderate heat. When the butter stops foaming, add the onions, carrots, celery, garlic and 1 teaspoon each of salt and pepper. Cook, stirring occasionally, until the vegetables are softened, about 8 minutes. Sprinkle the flour over the vegetables and stir until well coated, 1 minute. Add the tomato paste, meat, potatoes, chicken broth and milk and bring to a boil over moderately high heat. Reduce the heat to moderately low and simmer, stirring occasionally, until the potatoes are tender and the soup is slightly thickened, about 30 minutes.
Stir the 3 cups of cheese into the soup and cook until melted, about 2 minutes. Stir in 1 cup of the tomatoes and cook for 3 minutes, until softened and hot. Season with salt and pepper. Ladle the soup into bowls and garnish with the croutons, bacon, cheese, pickles, scallions and the remaining 1/2 cup of tomatoes. Serve.