Step 1

Place 4 tablespoons unsalted butter on the countertop to soften. Place 4 ounces cream cheese, 6 ounces shredded sharp yellow cheddar cheese (1 1/2 cups), 2 tablespoons whole grain mustard, 1/2 teaspoon of the kosher salt, and 1/2 teaspoon black pepper in a medium bowl and let sit out at room temperature while you prep the zucchini.

Step 2

Grate 1 medium zucchini (about 4 ounces) on the large holes of a box grater (about 1 1/4 cups). Place in a colander set inside a large bowl. Sprinkle with the remaining 1/2 teaspoon kosher salt, toss to combine, and let sit for 30 minutes. Squeeze the zucchini with your hands to remove excess water, then add to the bowl of cheese. Mix with a rubber spatula until combined.

Step 3

Butter one side of 8 slices marble rye bread with the softened butter. Flip 4 slices over and build the sandwiches: Using a butter knife, smear each with 1/3 cup of the cheese mixture. Close the sandwiches with the remaining bread slices buttered-side up.

Step 4

Heat a large skillet over medium-high heat. Add 2 of the sandwiches and cook until golden-brown and the cheese is melted, 3 to 5 minutes per side. Transfer to a cutting board and repeat with the remaining 2 sandwiches. Slice in half and serve immediately.