For the bread: Using a serrated knife, make a rectangular block of the bread by slicing off the ends and leveling the top and bottom of the loaf. There should be no crust left. Slice the block in half lengthwise, making 2 planks.
In a large bowl, whisk the heavy cream, chili powder, sriracha, eggs, a pinch of salt and a few grinds of pepper. Pour the mixture into a wide flat dish, large enough to dunk the bread. Carefully dip each plank in the egg mixture, coating all sides.
Heat a griddle over medium-high heat and melt the butter. Add the bread planks and cook on all sides until the batter is set and the bread is light golden, 4 to 5 minutes on each large side, and another 2 minutes for each edge.
For the filling: While still on the griddle, spread the raspberry jam on the entire top of one plank. Top with 4 slices of Gruyere, then all of the ham and turkey, shredding the meat as you go. Then cover with the remaining 4 slices of Gruyere. Like Jeff Mauro says, "crust to crust is a must!" In a small bowl, mix the mayonnaise and mustard together. Spread this mixture on the top of the remaining plank and lay it, mayonnaise-side down, on the other topped plank. Put an overturned metal bowl over the sandwich and cook a few minutes more to melt the cheese.
For the finish: Carefully remove to a baking sheet and allow to cool slightly. Dust with powdered sugar and use a serrated knife to cut 1 1/2-inch finger planks along the length. Serve with maple syrup to drizzle or dunk.