Prep the produce: Rinse and dry the spinach. Hull and slice the strawberries.
Slice the avocado just before serving to keep it fresh. Thinly slice the red onion.
Quick-candy the walnuts (no-stovetop hack): Toss walnuts with 1 tablespoon maple syrup and a pinch of salt. Spread on parchment and let sit 5–10 minutes to get tacky.
For extra crunch, air-dry longer or toast briefly if you don’t mind a 3-minute heat-up.
Make the vinaigrette: In a jar, combine balsamic vinegar, olive oil, Dijon, 1–2 teaspoons maple syrup, a pinch of salt, and pepper. Shake until emulsified. Taste and adjust sweetness or acidity.
Assemble the base: Add spinach to a large bowl.
Drizzle with half the vinaigrette and toss to lightly coat.
Add toppings: Layer on strawberries, avocado, red onion, and candied walnuts.
Finish and serve: Spoon on more vinaigrette as needed. Sprinkle a pinch of salt and pepper. Garnish with chopped basil or mint if using.
Serve right away.