For a perfectly indulgent and celebratory brunch, pair with Prosecco!
Grease a large baking dish with butter and set aside. Chop onion, remove stems from Swiss chard, and chop the leaves. Cube the bread. Grate the cheeses.
Place a large, heavy-bottomed sauté pan over medium heat; lightly oil pan, then add the sausages. Cook until browned and cooked through. Transfer to a paper towel-lined plate to drain and cool slightly, then slice.
Add more oil to pan if needed, then add onion and cook until soft and fragrant. Add chard to pan along with salt and pepper to taste and cook until wilted. Remove pan from heat and set aside.
Whisk together eggs, milk, mustard, nutmeg, and pepper to taste in a large measuring cup.
Spread half of the bread over bottom of baking dish and top with half of the sausage, half of the Swiss chard mixture, and half of the cheeses. Repeat, layering the remaining ingredients in this pattern. Pour egg mixture over everything, then cover tightly with plastic wrap and refrigerate overnight. Remove strata from refrigerator approx. 30 min. before baking. Preheat oven to 175°C/350°F. Bake for approx. 40 – 50 min., or until golden brown and cooked through. Let cool slightly before serving.