Preheat oven to 325°F. Place pecans on a rimmed baking sheet and bake until golden and fragrant, about 5 to 7 minutes, stirring halfway through. Transfer to a bowl and set aside to cool.
Mix spinach, strawberries, onion and goat cheese in a large bowl. In a separate bowl, whisk together balsamic vinegar, lemon juice and mustard. Add oil in a steady stream, whisking constantly, until combined. Season with salt and pepper. Toss salad with dressing, top with pecans and serve.