Pumpkin Grilled Cheese

Stir together pumpkin and mustard; season with salt and pepper. Spread 2 slices of bread with pumpkin mixture. Top with cheese and sage. Cover with remaining bread and brush both sides with butter. Cook in a large nonstick skillet over medium heat, flipping once, until golden and crisp, 8 to 10 minutes total. Serve, sprinkled with flaky salt.