Preheat the oven to 400°F (200°C).

In a medium bowl, blend the unsalted butter, cream cheese, sharp yellow cheddar cheese, whole grain mustard, 1/2 teaspoon kosher salt, and black pepper until smooth and creamy.

Carefully cut the medium zucchini into thin rounds.

Spread the cheese mixture generously over the slices of marble rye bread.

Arrange the zucchini rounds on each slice of bread, overlapping slightly for presentation.

Place the bread slices in the preheated oven and bake for about 25 minutes, or until the cheese is bubbly and the bread is golden brown.