Heat the butter in a 10-inch nonstick skillet over medium heat. Add the asparagus, bell pepper and shallots. Cook for 5 minutes or until vegetables are crisp-tender. Add the garlic and cook until just fragrant, about 30 seconds.
In a medium bowl, beat the eggs with salt and pepper; pour over the vegetables in the skillet. Reduce heat to low; cover and cook until the top of the egg mixture is set, about 8 to 10 minutes.
Place cheese slices evenly over the top of the frittata and heat another 1 minute until the cheese is melted. Remove from the heat, slice into wedges and serve immediately.
Source: Adapted from Sargento