Preheat oven to 400°F.
Line baking sheet with parchment paper; lightly coat with spray. Set aside.
Thinly slice cauliflower and shallot. Arrange cauliflower and shallot slices in single layer on prepared baking sheet. Coat with cooking oil spray, sprinkle with paprika, and season with salt and pepper to taste, if desired.
Bake for 20 minutes, or until cauliflower is tender. Set aside.
Spread mustard on one slice of bread. Layer cauliflower, shallot, arugula, and cheese; top with remaining slice of bread.
Heat oil in large nonstick skillet over medium heat.
Add sandwich; cook for 2 to 3 minutes, or until golden brown. Flip, then cook for 3 to 4 minutes, or until cheese has melted. Serve immediately.
The Nutrition Facts box below provides estimated nutritional information for this recipe.