In a small saucepan, bring the vinegar to a simmer over medium heat. Once the vinegar starts to simmer, reduce the heat to medium-low, and simmer for 6 to 10 minutes, uncovered, until the vinegar thickens, and reduces in volume by half. Set aside.
Meanwhile, add the strawberries, tomatoes, avocado, and basil into a large shallow bowl.
Drizzle on the oil and gently toss until the veggies are coated. Season generously with salt and pepper and toss again just briefly.
Scatter on your desired amount of toasted seeds or nuts. Drizzle on all of the balsamic reduction.
Serve immediately. This salad is definitely best enjoyed fresh, but leftovers can be stored in an airtight container for a few hours if necessary.