How to Make It
Heat the oven to 350°. Toast the pine nuts in the oven until golden brown, about 5 minutes. Toast the bread cubes in the oven until golden but still slightly soft in the center, about 15 minutes.
In a small glass or stainless-steel bowl, whisk together the vinegar, mustard, 1/2 teaspoon of the salt, and 1/4 teaspoon of the pepper. Add 5 tablespoons of the oil slowly, whisking.
Combine the flour with the remaining 1/2 teaspoon salt and 1/4 teaspoon pepper. In a large frying pan, heat the remaining 1 tablespoon oil and the butter over moderate heat. Dust the liver slices with the flour mixture and shake off the excess. Put the liver in the pan and cook until browned, about 3 minutes. Turn and cook until browned on the other side, 3 to 4 minutes longer. It should still be pink in the center. Remove.
In a large bowl, toss the spinach with all but 2 tablespoons of the dressing. Add the croutons and pine nuts and toss. Mound the spinach on plates and top with the liver. Drizzle with the reserved dressing.