Lightly grease four 12- to 16-ounce au gratin dishes or individual ramekins.
Gently pat the drained green chiles with paper towels to remove excess moisture. Slit one side of each chile, open it flat, and discard any seeds.
Pat dry again.
Arrange the chiles in a single layer in the bottom of your prepared dishes, cutting them as needed to fit.
In a medium skillet, melt the butter over medium heat.
Add the asparagus and sliced red onion, cooking for about 5 minutes until just tender-crisp, stirring occasionally. Spoon this vibrant mixture evenly over the chiles in each dish.
In a small bowl, toss the shredded cheese with the flour until well combined. This little trick prevents the cheese from sinking!
Sprinkle the cheese mixture evenly over the asparagus and onions.
In a separate medium bowl, whisk together the eggs, half-and-half, salt, and pepper until smooth. Carefully pour this custard mixture over the layers in each dish.
Cover the dishes with plastic wrap and refrigerate for at least 2 hours or up to 24 hours. This chilling step is key for the flavors to meld beautifully.
When you're ready to bake, preheat your oven to 350°F / 175°C.
Place the chilled au gratin dishes in a large 15x10-inch baking pan for easy handling.
Bake, uncovered, for 40 to 45 minutes, or until the centers are set and a knife inserted near the middle comes out clean.
Let the bakes stand for 10 minutes before serving. This allows them to set up perfectly.
Garnish with fresh chopped tomato and enjoy!
