Toast the sliced almonds in a dry skillet over medium heat, stirring frequently, until fragrant and lightly golden, 3–5 minutes. Transfer to a plate to cool.
Make the poppy seed dressing: In a mixing bowl, whisk together the apple cider vinegar, granulated sugar, Dijon mustard, kosher salt, and black pepper until the sugar dissolves. Slowly drizzle in the canola oil while whisking to emulsify, then whisk in the poppy seeds and the finely grated yellow onion.
Prep the salad components: Rinse and dry the baby spinach. Hull and slice the strawberries if not already done. Thinly slice the red onion. Crumble the feta cheese.
Assemble the salad: In a large bowl, combine the baby spinach, strawberries, red onion, most of the sliced almonds, and most of the feta cheese. Drizzle with about two-thirds of the dressing and gently toss to coat.
Finish and serve: Taste and add more dressing if desired. Top with the remaining sliced almonds and feta. Serve immediately.