Melt butter in a large skillet over medium heat. Add in the onions and salt, and cook until onions have caramelized, approximately 20 minutes. Remove from heat, add in the vinegar, stir to combine.
Meanwhile, place salmon cream cheese into a medium bowl, add in the lemon juice and dill, mix to fully combine. Place 1 1/2 teaspoons of salmon mixture into each phyllo shell, then top evenly with caramelized onions. Place tartlets onto a large baking sheet.
Place tartlets into preheated oven and cook for 5 - 7 minutes until heated through. Garnish with chopped fresh dill. Serve immediately.