Preheat your oven to 350°F. While it heats, generously coat a 9-inch pie plate with the softened butter, making sure to coat the bottom and sides evenly. Sprinkle the grated Parmesan cheese over the buttered surface, tilting the pan to distribute it evenly. This creates a flavorful crust layer that prevents sticking and adds a savory dimension to the pie.
In one bowl, whisk together the flour, baking powder, salt, pepper, and nutmeg—this creates your dry base that will give the pie structure. In a separate bowl, whisk together the eggs (they should be at room temperature for better incorporation), half-and-half, Dijon mustard, and melted butter until smooth and well combined. The mustard adds subtle depth without overpowering the delicate vegetables.
Combine the shredded Gruyere, trimmed asparagus pieces, and thinly sliced scallions in a bowl, then distribute this mixture evenly into the prepared pie plate. I like to lightly press down the mixture so it sits compactly and ensures the custard will seep through all the layers for even cooking. This foundation will catch and distribute the custard as it sets.
Pour the wet mixture from Step 2 over the vegetable and cheese layer in the pie plate. Gently stir the wet and dry ingredients together directly in the pan to create a uniform custard batter—don't overmix, just combine until no dry streaks remain. Sprinkle the sriracha seasoning over the top and use a fork to gently incorporate it throughout. The gentle mixing keeps the pie tender rather than dense.
Bake the pie in your preheated 350°F oven for 30 minutes, until the custard is set in the center but still slightly jiggly when gently shaken—it will continue cooking slightly as it cools. The pie is done when a toothpick inserted in the center comes out clean or with just a few moist crumbs.
Remove the pie from the oven and let it rest on the counter for 15 minutes before slicing. This resting period allows the custard to set properly so slices hold together beautifully. I've found that skipping this step often results in a pie that falls apart when serving. Top each slice with a pinch of lemon zest and fresh minced chives just before serving for brightness and a fresh herbaceous note.