01 - Preheat oven to 190°C (375°F). Grease a 23x33 cm (9x13 inch) baking dish with olive oil.
02 - Heat olive oil in a skillet over medium heat. Add chopped asparagus, onion, and red bell pepper, sautéing until vegetables are crisp-tender, about 4-5 minutes.
03 - In a large bowl, whisk eggs with milk, salt, and pepper. Stir in shredded cheddar cheese.
04 - Spread sautéed vegetables evenly in the prepared baking dish. Pour the egg and cheese mixture over the vegetables, ensuring even distribution.
05 - Transfer to the oven and bake for 22-25 minutes, or until the center is just set and the edges are lightly golden.
06 - Allow the baked frittata to cool in the dish for 5 minutes. Cut into squares and serve warm or at room temperature.