Real maple syrup—not the "maple-flavored" fakes that line the supermarket shelves—is the key to the richly flavored dressing on this elegant salad. In this case, the darker the syrup the better; choose Grade B syrup if it’s available, or the darkest "Grade A" that you can find.
Place chicken in a small skillet or saucepan and add enough water to cover; bring to a simmer over high heat. Cover, reduce heat and simmer gently until the chicken is cooked through and no longer pink in the middle, 10 to 12 minutes. Transfer the chicken to a cutting board. When cool enough to handle, cut into 1/4-inch-thick slices.
Meanwhile, whisk oil, syrup, vinegar, mustard, soy sauce, salt and pepper in a large bowl. Reserve 2 tablespoons dressing in a small bowl. Add spinach to the large bowl; toss to coat with dressing. Divide the spinach between 2 plates, top with the chicken, beets, goat cheese and pecans. Drizzle with the reserved vinaigrette.