Make the dressing: In a small mixing bowl, add olive oil, balsamic vinegar, mustard, salt and pepper and whisk vigorously until completely combined.
In a large mixing bowl, toss greens, strawberries, apple, and red onion together. Pour about half of the dressing over the salad mixture and toss to combine.
Sprinkle almonds and feta on top and drizzle with the rest of the dressing.
Serve immediately.