Preheat your oven to 400°F (200°C).
Prepare the acorn squash by halving it, removing seeds, and cutting into 1-inch cubes.
Toss the cubes of acorn squash in olive oil, salt, and pepper, then spread them evenly on a baking sheet.
Roast the acorn squash for about 25-30 minutes, or until tender and golden brown, tossing halfway through.
While the squash roasts, prepare the dressing by whisking together olive oil, apple cider vinegar, maple syrup, salt, and pepper.
In a large bowl, combine the mixed greens, sliced apple, red onion, and walnuts.
Once the squash is roasted, allow it to cool for a couple of minutes before adding it to the bowl with the greens.
Drizzle the dressing over the salad and toss gently to combine all ingredients.
Top with crumbled feta cheese, if using, and serve immediately.