Grease a 9 x 13-inch casserole dish or spray with nonstick spray
Arrange cubed bread in a single layer in the pan and set aside
Cook the sausage in a large skillet over medium heat, breaking it apart. When cooked through, remove from the skillet and drain off the grease. Sprinkle over the bread cubes.
In the same skillet over medium heat, add a tablespoon of olive oil and add the onion. Cook until soft and translucent (about 4 minutes), stirring occasionally. Add the garlic and the Swiss chard and cook until it is wilted (about2-3 minutes). Remove from the heat and spoon evenly over the bread/sausage, then sprinkle all the cheese on top
In a large bowl, whisk the eggs, milk, salt, ground mustard, and salt and pepper together. Pour evenly over the entire casserole and press down on the bread, ensuring the bread is absorbing the egg/milk mixture. Cover and refrigerate for 1 hour or overnight (up to 12 hours)
Remove the strata from the refrigerator and allow it to sit out at room temperature for 15 to 20 minutes while the oven is preheating.
Preheat oven to 375℉
Bake the strata, uncovered, for 20 minutes then loosely cover with aluminum foil and bake for an additional 30 minutes. The strata will be browned on the sides and buffy in the center.
Remove from the oven and allow to cool for 5 minutes before serving