Place onion slices in medium bowl. Combine vinegar, water, sugar, and kosher salt in small saucepan and bring to boil over high heat, stirring to dissolve sugar. Pour hot pickling liquid over onions and cover bowl with plastic wrap. Cool. Onions can be refrigerated for up to 2 weeks.
Heat sauté pan over medium heat, add 2 tablespoons of olive oil and 4 slices of bread and sauté, pressing bread down with a spatula. When bread begins to lightly crisp, on each slice place 1 ounce Kasseri, some spinach leaves, 1 slice prosciutto, and a few tablespoons of pickled onions. Top with another slice of bread and repeat layer of Kasseri, spinach leaves, prosciutto, and pickled onions. Top with remaining bread slices. Carefully turn sandwiches over; press together and cook until bread is browned and cheese is melted.
- 2 red onions, thinly sliced
- 1/2 cup red wine vinegar
- 1/2 cup water
- 1/4 cup sugar
- 1 tablespoon kosher salt
- 2 tablespoons extra virgin olive oil
- 12 slices artisan-type rosemary-olive bread
- 8 ounces fresh spinach leaves
- 8 ounces Wisconsin Kasseri cheese, shaved or shredded
- 8 thin slices prosciutto