Lightly grease a 9x13 baking dish with cooking spray.

Lay out about 6 slices of sourdough on your work surface; you’ll make around 6 sandwiches from the loaf.

Spread each of the bottom slices generously with red raspberry preserves, covering the surface.

Layer a hearty amount of thinly shaved ham on each slice (Mary Catherine starts with about 3 slices per sandwich and adds 1 more; use most or all of the 16 oz).

Top the ham on each with 1 slice Swiss cheese.

On the remaining bread slices, squirt on a generous amount of honey mustard and spread to the edges; don’t skimp, this adds a lot of flavor.

Place the honey‑mustard slices on top to make full sandwiches.

Slice each sandwich in half lengthwise.

Cut each long half into about 4 triangles, so you end up with small sandwich wedges.

Arrange the triangles upright in the greased 9x13 dish with the points standing up, wedging them in snugly; the dish will be very full and doesn’t need to look perfect.

In a bowl, crack 8 eggs and whisk until slightly beaten.

Add 1¾ cups milk, the kosher salt, and black pepper, then whisk until well combined.

Slowly pour the egg mixture evenly over all the sandwich pieces, letting it run between and around the triangles.

Sprinkle about 4 oz shredded Gruyère over the top (you don’t need the full 6 oz bag).

Cover the dish tightly and refrigerate at least a few hours, overnight, or up to 2 days so the bread can soak up the custard.

When ready to bake, preheat the oven to 350°F.

Uncover the casserole and bake for about 35 minutes, until the custard is mostly set and the top is golden and bubbly; depending on your oven, you may want to go 3–5 minutes longer if the center seems a little soft.

Let the casserole rest about 10 minutes after baking.

Dust the top with a light coating of powdered sugar.

Cut into portions and serve warm with a spoon or spatula, alongside dollops of red raspberry jam (and hot pepper raspberry jam if desired) for dipping.