Make slices almost to bottom of loaf but leaving 1/4 inch or so, about 1-inch apart, on a slant. Then, do this again but on the opposite slant, so you’re making little diamonds across the loaf while leaving bottom intact.
Stuff thin slices of cheese in openings. Place loaf on foil cookie sheet. In a pan, melt butter and sauté mushrooms until caramelized. Off the heat, stir in the onion, poppy seed, salt, mustard, and lemon juice. Pour mixture over bread.
Seal foil tightly around loaf. Bake at 350°F for 45 min on top oven rack.