Directions
Preheat your oven to 300°F / 150°C.
In a large bowl, whisk together the eggs, ricotta cheese, lemon juice, salt, and pepper until smooth and well combined.
Heat the olive oil in a 10-inch oven-safe skillet (cast iron works perfectly) over medium heat.
Add the sliced onion, green pepper, and mushrooms, and sauté for 5 minutes until they begin to soften.
Add the cut asparagus pieces to the skillet and cook for an additional 3 minutes, stirring occasionally, until the asparagus is tender-crisp and bright green.
Pour the egg mixture directly into the skillet over the cooked vegetables. Give it one gentle stir to distribute everything evenly.
Transfer the entire skillet to the preheated oven and bake for 20-25 minutes, or until the frittata is puffed and the center is just set.
Carefully remove from the oven and let the frittata rest in the skillet for 5 minutes. This allows it to set fully, making it easier to slice.
Cut into 8 wedges and serve warm.
Don't overbake the frittata.
Remove it from the oven when the center has a very slight jiggle; it will continue to cook with the residual heat from the skillet.