Pour the balsamic vinaigrette into the bottom of a clean 32-ounce wide-mouth mason jar.
Add the sliced strawberries and blueberries as the second layer to provide a barrier between the dressing and the greens.
Add the crumbled goat cheese and chopped pecans as the third layer.
Pack the baby spinach tightly into the remaining headspace of the jar.
Secure the lid tightly and refrigerate at 40 degrees Fahrenheit or below until consumption.
Invert and shake the jar or decant into a bowl immediately prior to serving to distribute the dressing.