Butter a large casserole dish with 2 tablespoons (30g) butter, using more if needed. Set aside.

Prepare the sausage according to package directions. Cut into bite-sized pieces, and set aside.

Melt 1 tablespoon butter in your largest skillet. Add the chopped onion and cook until it begins to brown.

In the same skillet, add the remaining tablespoon butter and olive oil. Over medium-high heat, sauté the mushrooms, stirring occasionally, allowing them to fully release their juices and brown.

When the mushrooms are nearly done, add the asparagus. Sauté for 1 minute; the asparagus will be bright green but still crisp. Stir in the sausage, remove from heat, and set aside.

In a medium bowl, whisk the eggs, cream, nutmeg, and salt together to make the custard, and set aside.

Now assemble the casserole. Layer the casserole, beginning with ⅓ of the bread, ⅓ of the vegetables and sausages, and ⅓ of the cheese. Continue to layer, reserving ⅓ of the cheese until after the custard is added.

Pour the custard evenly over the casserole, pressing the bread down with a spatula or back of a spoon to ensure that all of the bread cubes are submerged. Top with the remaining shredded cheese and the chives, cover tightly with plastic wrap, and refrigerate for several hours or overnight.

Once ready to bake, remove the plastic wrap and preheat your oven to 350ºF (175ºC) or 325ºF (160ºC) convection bake. Place the casserole on a baking dish and bake for 60 minutes, or until slightly puffed and set in the center. Allow to cool for several minutes, and serve warm.