Chicken Pot Pie Soup

3 chicken breasts, skin-on, bone-in (2½ to 3 pounds total)

Good olive oil

Kosher salt and freshly ground black pepper

6 tablespoons (¾ stick) unsalted butter

5 cups chopped leeks, white and light green parts (3 leeks) (see note)

4 cups chopped fennel, tops and cores removed (2 bulbs)

3 cups (½-inch) diced scrubbed carrots (5 medium)