Chop any woody ends off asparagus and cut it into 0.5 cm (1/4 inch) chunks. Clean the spring onion; use only the green part. Cut it into 0.5 cm (1/4 inch) chunks.
pan fry the vegetables
Place a 24cm or 9.5-inch skillet over medium heat. Add the olive oil, asparagus, and diced onion. Pan fry for 5 minutes, stirring occasionally.
whisk the eggs
Add eggs to a bowl and season with salt and pepper. Whip the eggs until creamy. Add the mascarpone and gently scramble the mixture. There should be bigger pieces of mascarpone visible still.
make the frittata
Pour the egg mixture into the asparagus and divide evenly. Place back on medium heat. Using a spatula, gently stir the eggs for about 2 minutes or until the eggs start to set in the bottom of the skillet. Then cook for another minute, pushing the eggs toward the center of the pan. They should get almost scrambled. Reduce the heat to low heat. Shake the pan, cover with a lid and cook for 6 - 7 minutes. Flip the frittata. You can cut the frittata in half and flip each half, or you can flip the whole frittata onto a plate, turn it, and then place it back in the skillet. Cook for another 2 minutes on the other side.
Cut the frittata into slices and divide them between four plates. Optionally sprinkle with fresh herbs like chopped parsley or chives and season with salt.