Wash cherry tomatoes and halve them. Peel (if desired) and thinly slice the cucumber into half-moons to total about 1/2 cup. Thinly slice the red onion into half-rings and separate. Add tomatoes, cucumber and red onion to the bowl with spinach.

Toast walnuts

In a small dry skillet over medium heat, add the walnuts and toast, stirring frequently, until fragrant and lightly browned, about 3–4 minutes. Watch closely to avoid burning. Remove from heat and let cool slightly, then roughly chop if pieces are large. Add walnuts to the salad bowl.

Add cheese & avocado

Crumble the feta and sprinkle over the salad. If using avocado, halve and pit it, scoop out the flesh and slice or dice it; add to the bowl just before dressing to avoid browning.

Dressing

Make dressing

In a small jar or bowl, combine the extra-virgin olive oil, fresh lemon juice, Dijon mustard, honey (or maple syrup), red wine vinegar, minced garlic, salt and black pepper. Whisk vigorously or secure the jar lid and shake until the dressing is emulsified and slightly thickened, about 20–30 seconds.

Adjust seasoning

Taste the dressing and adjust: add more lemon or vinegar for acidity, more honey for sweetness, or a pinch more salt and pepper to preference.

Dress the salad

Pour about half the dressing over the salad and toss gently to combine, coating the leaves and other ingredients evenly. Add more dressing as needed to taste (you may not need all of it).

Serve

Transfer the dressed spinach salad to plates or a serving bowl. Serve immediately so the spinach stays crisp. Leftover dressing can be refrigerated for up to 5 days; toss salad just before serving to avoid sogginess.