Combine 1 cup feta, ½ cup yogurt and 1 tablespoon oil in a medium bowl. Mash with the back of a fork or a potato masher until creamy and well combined.
Spread about 2 tablespoons feta mixture on 1 side of each of 8 toast slices. Top 4 slices with sliced cucumber and sliced avocado. Sprinkle the avocado with ¼ teaspoon salt. Top with 1 cup spinach and the onion rings. Cover with the remaining toast slices, feta-sides down; press lightly.
