For the mustardy dressing: Combine the mustard and vinegar in a small bowl, then whisk in the oil until emulsified. Season to taste.
For the salad: Preheat the oven to 425 degrees F.
Toss the squash in the olive oil and spread out on a rimmed baking sheet. Sprinkle with a few pinches of salt, a few turns of pepper and the cayenne. Roast until browned, tossing halfway through, 18 to 20 minutes. Allow to cool.
In a small bowl, toss the apple with the lemon juice.
Place the spinach in a large serving bowl, then add the toppings beautifully in rows: the squash, onion, corn, Cheddar, sunflower seeds and the apple. Dress as desired.