3 tbsp (45 mL) unsalted butter
2 cups (500 mL) yellow onion, cut into slices ½ inch (1 cm) thick
Sea salt, to taste
1/3 cup (75 mL) mayonnaise
1 tbsp (15 mL) ketchup
1 tbsp (15 mL) relish
Pinch of pepper
8 slices of dark rye bread
4 large slices of Gruyère cheese (approximately 3½ oz/100 g)
1 cup (250 mL) prepared sauerkraut
Heat a large skillet on low heat and melt the butter. Stir in the onion and a pinch of salt and cover. Cook for approximately 20 minutes, stirring occasionally. Remove the lid and continue to cook and stir occasionally for another 20 minutes or until the onion is soft and translucent. Remove from the heat and set aside to cool.
Mix the mayonnaise, ketchup and relish in a small bowl. Season with salt and pepper, to taste. Set aside.
Toast 2 pieces of bread and spread 1 tbsp (15 mL) of sauce on each piece. Place 1 slice of Gruyère cheese on 1 slice of the toasted bread, along with a scoop of sauerkraut and caramelized onion. Top with the remaining piece of toasted bread. Cut in half if desired and serve immediately. Repeat with remaining ingredients.