Preheat your oven to 350°F (175°C).
Heat 1 tbsp of olive oil in an oven-safe skillet over medium heat.
Add the chopped onion and cook until softened, about 5 minutes.
Add the asparagus pieces and cook for another 3-4 minutes until they are bright green and slightly tender.
In a bowl, whisk together the eggs, milk (or cream), cheese, salt, and pepper.
Pour the egg mixture into the skillet over the vegetables. Stir gently to distribute the ingredients evenly.
Cook on the stovetop for about 5 minutes, until the edges start to set.
Transfer the skillet to the preheated oven and bake for 10-15 minutes, or until the frittata is fully set and slightly golden on top.
Remove from the oven and let cool slightly before slicing and serving.