For the dressing, combine the vinegar, sugar, lemon juice, and salt in a small non-reactive saucepan and heat over medium heat until the sugar dissolves, stirring frequently. Remove pan from heat and let cool to room temperature. When cooled, whisk in the oil, onion, poppy seeds, dry mustard and paprika until thoroughly combined. Set dressing aside. In a salad bowl, combine the spinach, hearts of palm, strawberries, and walnuts. When ready to serve, add some of the dressing, and toss gently. Serve the remaining dressing alongside the salad so diners may add more, if desired.