1 tablespoon olive oil, plus more for garnish
1 slice thick-cut smoky bacon
1/2 large onion, chopped
1 medium piece celery, chopped
1 large carrot, peeled and chopped
Salt and pepper to taste
1 teaspoon chopped fresh rosemary (optional)
1 14-ounce can diced tomatoes, a few tablespoons set aside
3 15-ounce cans cannellini beans, 2 cans with their liquid and 1 can drained and set aside
1 cup homemade chicken stock or unsalted store-bought chicken broth, plus a little extra for thinning out