Preheat the oven to 350˚F.

Spread the pumpkin seeds onto a rimmed baking sheet and bake for 8-10 minutes, until lighly golden and fragrant. Transfer to a dish to cool.

Place the red onions on a baking sheet and toss with 1 tablespoon of the olive oil. Roast in the oven for 10-15 minutes, stirring once partway through the cooking time. Transfer to a large serving bowl and allow to cool.

Place the butternut squash on a baking sheet, toss with 2 tablespoons of the olive oil, and sprinkle with salt and pepper. Roast 20-25 minutes, until tender. Add the squash to the bowl with the onions, and set aside to cool.

In a small mixing bowl, whisk together the Dijon mustard, red wine vinegar, 1/2 teaspoon of salt, and a generous pinch of black pepper. In a slow, steady stream, drizzle in the remaining 3/4 cup olive oil, whisking until emulsified. Taste and add more salt and pepper if needed.

Fold the baby spinach into the bowl of squash and onions. Drizzle with red wine vinaigrette, and toss to coat. Serve topped with the toasted pumpkin seeds.