Russian dressing:
2 tablespoons reduced-fat mayonnaise
2 teaspoons ketchup
2 teaspoons chopped capers
1 teaspoon chopped pickle or relish
Sandwiches:
3 teaspoons extra-virgin olive oil, divided
1 small red onion, thinly sliced
1 cup sliced mushrooms
5 cups baby spinach
Freshly ground pepper to taste
4 slices rye bread
1/2 cup shredded reduced-fat Swiss cheese, such as Jarlsberg Lite or Alpine Lace (2 ounces)
1/2 cup sauerkraut
Prepare Russian dressing: Whisk mayonnaise and ketchup in a small bowl until smooth. Stir in capers and pickle (or relish).
To prepare sandwiches: Heat 2 teaspoons oil in a large nonstick skillet over medium-high heat. Add onion and mushrooms; cook, stirring often, until the onion is softened, 4 minutes. Add spinach and cook, stirring, until it has wilted, 1 to 2 minutes. Transfer the mixture to a plate.
Coat the pan with the remaining 1 teaspoon oil and return to medium heat. Add the bread; divide cheese equally among the slices. Divide sauerkraut between 2 slices and divide the spinach mixture between the other 2 slices; cook until the cheese has melted and the bread is golden brown, 4 to 6 minutes. Transfer sandwich halves to a cutting board. Divide the dressing between the spinach halves. Carefully place the sauerkraut halves on top. Cut sandwiches in half and serve.
