Peel butternut squash and cut off the ends. Lie squash on its side and carefully cut off the neck just above where thesquash begins to widen. Cut bulb of squash in half, scoop the seeds out of each half, and discard. Cut squash into 1” dice. Toss cubed squash with 3tbsp olive oil, salt, and pepper, spread in single layer on sheet pan, and roast until the squash is tender and browned, about 30-35 minutes. Cool.
Peel onions and cut off the ends. Cut in half lengthwise, then cut each half into six wedges. Toss with 1tbsp olive oil and spread in single layer on sheet pan (line pan with parchment paper or spray lightly with cooking spray for easier cleanup!). Roast onions in preheated oven until soft and browned around the edges, about 20 minutes. Shake pan or stir onions if they are browning too quickly. Cool.
Put mustard and vinegar in a large bowl and whisk to combine, then slowly add in olive oil while whisking constantly to make a creamy dressing. 5.Add cooled squash and onions and toss to coat.
Add spinach and pumpkin seeds and toss again gently to avoid bruising the spinach. Season with more salt and pepper to taste.