Prepping the Strata (best done the night before serving):

1. Butter or spray a 9 x 13-inch baking dish.
2. Cube your pretzels and place them in the dish.
3. In a skillet set over medium heat, crisp up the bacon. Remove from the pan with a slotted spoon and sprinkle the bacon over the pretzels in the baking dish, distributing it evenly. If preferred, you can save back a few pieces for garnishing the top (unnecessary but nice for presentation).
4. To the bacon drippings in the skillet, cook the onion with a pinch of salt. Cook until softened, about 5-7 minutes. Distribute the onions in the pan with the pretzels, just as you did with the bacon.
5. In a large mixing bowl, whisk together the eggs, half-and-half, mustard, salt, and pepper until well combined.
6. Sprinkle half the cheeses over the pretzels in the pan. Pour the egg mixture over everything, distributing it as evenly as you can.
7. Gently press the pretzels down into the egg custard so that everything is evenly submerged.
8. Cover the dish with plastic wrap and transfer to the fridge to set for at least 4 hours (longer is better – I try to do this the day before I want to serve and have it set in the fridge overnight).

Baking the Strata:

1. Remove the baking dish from the fridge and preheat the oven to 350°F.
2. Sprinkle the remaining cheeses over top of the strata (as well as any bacon you may have reserved).
3. Bake for 50-60 minutes, or until the custard is set and the top is golden brown and crisp in spots. Allow to cool slightly before serving.
4. Sprinkle over fresh chives and flaky salt right before serving.