Wash and dry the strawberries with kitchen paper. Top and slice them. Place them in a small bowl, toss with the sugar and marinate for about 30 minutes to 1 hour. Take the strawberries out of the marinade, but reserve the marinade to make the dressing.
Toast the almonds in a large pan without adding any fat. Stir very often and remain near the pan, if unattended the thin almond flakes will burn easily. Once roasted, transfer the almond to a large plate and let cool while you prepare the rest of the salad.
Make the dressing. Whisk together the olive oil, balsamic vinegar, about 2 tablespoons of the reserved strawberry marinade, mustard, mayo, salt, and pepper to taste. Stir in the poppy seeds. Adjust the taste with more balsamic vinegar, strawberry marinade, and salt and pepper, if necessary.
Wash and dry the baby spinach leaves. Place in a bowl. Add about ½ of the poppy seed dressing and toss with your hands making sure that all the spinach leaves are coated with the dressing. Arrange the spinach on two plates or a larger serving platter.
Arrange the strawberry slices on the spinach bed.
Finely chop the red onions and slice the avocado. Arrange them both on the plate. Crumble the feta cheese on top.
Pour the rest of the poppy seed dressing on top of the salad.
Scatter the toasted almonds on top of the salad, garnish with some basil leaves, and serve immediately.