1. Lightly coat 1 side of each bread slice with mayonnaise. Coat a medium sauté pan with cooking spray and place over medium-low heat. Add bread slices to pan and toast until crisp, about 2 minutes per side. Remove pan from heat and transfer toast to a clean work surface. Cut each slice in half on the diagonal.
2. Arrange 2 or 3 avocado slices on each piece of toast. Add onion slices, then fold smoked salmon on top. Add a dollop of Greek yogurt and sprinkle with caraway seeds. Garnish with fresh oregano, if desired. Serve open-faced.