For dressing: Zest lemon to measure ½ teaspoon zest, juice lemon to measure 2 tablespoons juice. Whisk / emulsify together vinegar, zest, juice, sugar, oil and poppy seeds. Whisk until blended. See How to make the Perfect Vinaigrette Cover and refrigerate.
Hull strawberries and cut into quarters. Cut cucumber in half lengthwise; remove seeds and slice. Place spinach in large serving bowl. Add strawberries, cucumber and onion. Whisk dressing; pour over salad. Gently toss to cover.