Gather all ingredients and preheat your oven to 350°F (175°C). Lightly grease a 9-inch pie pan to prepare it for baking.
Heat the vegetable oil in a large skillet over medium-high heat. Add the chopped onion and sauté, stirring occasionally, until softened, about 3 minutes. Incorporate the spinach and continue cooking until any excess moisture evaporates completely.
In a large mixing bowl, whisk together the beaten eggs, salt, and pepper. Add the shredded Muenster cheese and the spinach-onion mixture, stirring until the ingredients are thoroughly combined.
Pour the mixture into the prepared pie pan, spreading it evenly.
Bake in the preheated oven for approximately 30 minutes, or until the eggs are fully set and the frittata is golden and slightly puffed.
Remove from the oven and allow the frittata to cool for about 10 minutes before slicing and serving.
Serve warm and enjoy this flavorful dish!