Preheat oven to 375 degrees F. Set out an 11 x 7 baking dish.

Start with the vegetable prep. Snap the woody ends off the asparagus (about 1/3 of the way from the bottom). Soak the asparagus top down in a big bowl of cold water and swish several times to get any sand out of the tips. Pat dry. If spears are very thick or look fibrous, use a peeler to lightly remove the outermost layer.

Trim the root end from the spring onions. Cut off the dark green part and reserve for garnish. Rough chop the purple and white section.

Arrange asparagus and spring onions in baking dish.

Set a medium-sized saucepan over medium heat. When hot, add 2 tablespoons butter. Once butter has melted, fold in the shallot, nutmeg, and pepper, and cook 1 minute. Stir in the flour, and cook 2 minutes more, stirring all the while. Edge heat down to medium-low and slowly add the milk. Stir until the mixture is quite thick, about 5 minutes. Remove from heat, fold in the cheese, and stir until cheese is melted.

Pour cheese mixture over the vegetables, nudging the asparagus as needed to get everything coated in cheese sauce. Bake about 30 minutes.

While the gratin bakes, set a small skillet over medium heat. Add the remaining tablespoon butter, and when hot, add the panko. Cook, stirring frequently, until breadcrumbs are a deep golden hue. Remove from heat, fold in a teaspoon minced chives, and set aside to cool.

When the gratin has about 10 minutes left to cook, remove from oven, top with toasted panko, and cook until the gratin is bubbling and the edges of the cheese sauce have turned golden brown.

Garnish with chives and chive blossoms, chopped spring onion greens, or the herbs of your choice. Serve immediately.