Combine the cucumbers, onion, sweetener and vinegar in a small bowl.
Remove the flesh from the avocados and chop into 1/2 inch pieces.
Assemble the stacks: Spread out 2 cups of spring greens on a salad plate. Pack one quarter of the chopped salmon into a 4 inch ramekin or dish. Top the salmon with one quarter of the cucumber onion mixture, then one quarter of the chopped avocados. Press the stack down gently to compact the layers but do not mash them out of shape. Carefully turn the ramekin over on the salad greens to unmold the stack.
Repeat Step 4 until you have four complete stacks. Drizzle 2 tbsp of dressing over each stack and serve.
Alternate Method than Stacking: If you don't have a ramekin or don't want to bother with stacking the ingredients, you can spread the greens on a large platter and then scatter the toppings over them and serve the dressing on the side.
Notes
Nutrition Info is for the smoked salmon stacks only. Dressing is your choice and would need to be counted in as well. You can choose just lemon juice over the top if desired.