Use Hokkaido roasted pumpkin (see my blog for how to roast it), or cook the pumpkin cubes in an 8-inch (20 cm) skillet with a lid over medium heat for about 10 minutes, until tender and golden. Let cool slightly.
Spread the spinach and arugula in a deep bowl (about 7 inches / 18 cm wide).
Top with the cold pumpkin and halved cherry tomatoes.
Right before serving, whisk the Dijon mustard, olive oil and balsamic vinegar (or lemon juice) in a small cup. Sweeten with honey or maple syrup if you like.
Drizzle with the dressing and optionally scatter pumpkin seeds on top.