Preheat oven to 350°F.
Bring a pot with a couple inches of water in it to a boil. Place the sweet potatoes in a steamer basket over the water, and cover and steam them for 5 minutes. Transfer the sweet potatoes to a baking sheet in a single layer and roast just until lightly browned on the edges, about 35-45 minutes. Set aside to cool.
Place the red onions on a baking sheet and roast until tender and browned, about 20 minutes. Set aside to cool.
Spread the pumpkin seeds on a rimmed baking sheet and toast for 6-7 minutes. Set aside to cool.
Whisk together the vinegar and mustard in a small bowl. While whisking, drizzle in the oil in a steady stream and continue to whisk until emulsified. Whisk in salt and pepper.
When the roasted vegetables have cooled to room temperature, place them in a large bowl. Add the baby spinach and drizzle with some vinaigrette. Toss to coat, and sprinkle with the toasted pumpkin seeds. Serve with any extra vinaigrette.