1. Place onions and sugar in a large pot over medium heat and stir until the sugar begins to dissolve.
2. Cover the pot and allow the onions to cook at a low boil for 15 minutes, stirring occasionally, until the onions are soft and translucent.
3. Add the lemon juice and stir to combine.
4. Neatly lay the salmon slices over the onion mixture.
5. Cover the pot and allow the fish to cook for 20 minutes. Remove the pot from the heat.
6. Transfer the fish slices to a 10- by 15-inch (25- by 40-centimeter) pan.
7. Top fish with the onion mixture. Seal with foil and refrigerate.
8. Serve with Avocado Ranch Dressing.