Prep the bread: Cube the sourdough loaf and leave it exposed to ambient air for 24 hours to stale, or lightly toast it in a low-temperature oven to remove excess moisture.
Cook the sausage: Heat a heavy-bottomed skillet over medium-high heat. Brown the sausage, breaking it into uniform morsels. Drain thoroughly on paper towels to remove all excess grease.
Whisk the custard: In a large mixing bowl, combine the eggs, whole milk, dry mustard, garlic powder, onion powder, salt, and black pepper. Whisk vigorously until pale and highly aerated.
Layer the ingredients: Lightly grease a 9x13-inch baking dish. Arrange the desiccated bread cubes across the bottom. Evenly distribute the browned sausage over the bread, followed by an even layer of the grated cheeses.
Combine: Slowly pour the seasoned egg custard over the entire assembly. Gently press down on the solid components to ensure total submersion and hydration.
Chill overnight: Tightly secure the baking vessel with plastic wrap and refrigerate for 4 to 12 hours so the bread acts as a sponge and absorbs the liquid.
Bake: Let the chilled dish sit at room temperature for 30–45 minutes. Preheat your oven to 175°C (350°F). Bake uncovered for 45–55 minutes. If the top begins to over-brown, loosely tent the dish with aluminum foil.
Finish: Verify doneness with a digital thermometer (the geometric center must reach 71°C/160°F). Garnish with freshly chopped scallions before serving.
