To prepare the Poppy Seed Dressing, combine the lemon juice, oil, honey, mustard and salt in a small jar with a lid. Shake it vigorously until the dressing looks emulsified, then taste and adjust any seasoning as needed. Add the poppy seeds and shake again, then set it aside for the flavors to meld.
To assemble the salad, place the spinach in a large serving bowl, then top it with strawberries, red onion, and avocado. Drizzle the dressing over the top and gently toss, just before serving.
If you plan to save this salad for later, store the salad ingredients separately from the dressing for best shelf life. The dressing should last up to 5 days in an airtight container the fridge.