- With the rack in the middle position, preheat the oven to 425°F (210°C). Line a baking sheet with parchment paper.
- In a large non-stick skillet over medium heat, soften the onions in 2 tbsp (30 ml) of the oil for 30 minutes or until translucent and lightly browned. Season with salt and pepper. Keep warm.
- Meanwhile, place the bread slices on the prepared baking sheet. Oil lightly. Toast for 10 to 12 minutes or until the bread begins to brown.
- Spoon the onions onto each toast. Add a layer of sour cream and sprinkle with the sumac. Cover with the asparagus and feta cheese.
- In a bowl, combine the parsley, the remaining oil (1 tbsp / 15 ml) and the lemon juice. Top each toast with the parsley mixture.