Roast the cauliflower: Preheat the oven to 450 degrees F. Cut off the cauliflower stem, then place the head cut-side down and slice into 1/2-inch-thick steaks. In a small bowl, whisk together the olive oil, mustard powder, paprika and some salt and pepper. Brush the cauliflower steaks with the spiced oil. Place them flat on a baking sheet and roast for 10 minutes. Carefully flip the cauliflower, return to the oven and roast until golden, another 10 to 12 minutes.
Meanwhile, make the honey mustard: In a small bowl, mix together the yellow mustard, stone-ground mustard and honey. Season with salt and pepper. Set aside.
For the sandwich build: Melt 2 tablespoons butter on a griddle over medium-low heat. Spread some honey mustard on 4 pieces of bread; top each with some cheese, roasted cauliflower, sliced pears, more cheese and another piece of bread. Place 2 sandwiches on the griddle and cover with a metal bowl. Cook until golden and crisp on the outside and gooey in the middle, 4 to 5 minutes per side. Add the remaining 2 tablespoons butter to the griddle and repeat with the remaining sandwiches. Serve with the remaining honey mustard.