Preheat the oven to 425 degrees F. Peel squash, remove seeds, and cut into 1-inch cubes. Place onto a baking sheet and drizzle with olive oil. Season with salt and pepper and toss so squash is evenly coated. Roast for about 20-25 minutes, until squash is soft and starting to brown around the edges.
While squash roasts, make dressing. In a small jar, add olive oil, apple cider vinegar, dijon mustard, honey, sea salt, and black pepper. Shake until emulsified.
Assemble salad by placing spring mix in a large salad bowl or platter and drizzle with dressing to taste. (Start on the lighter side so everyone can add their own perfect amount.) Gently toss greens and dressing and top with sliced chicken, roasted squash, feta cheese, and roasted pepitas.
Season with flaky sea salt and freshly cracked black pepper to taste and serve immediately.