Heat olive oil in a skillet over medium heat and cook spinach for 2 to 3 minutes until wilted. Transfer to a sieve, cool briefly and squeeze firmly to remove excess moisture, then chop.
Mix spinach with drained ricotta, chopped sun-dried tomatoes and black pepper until evenly combined.
Divide half the Muenster over 2 bread slices, spread the spinach mixture evenly over it and add remaining cheese. Close with the other bread slices.
Butter the outside faces and cook in a clean large skillet over medium-low heat for 4 to 5 minutes per side, covering for 2 minutes after turning, until golden and the Muenster is melted throughout.
Rest for 2 minutes, cut each sandwich in half and serve.