Line sliced zucchini in an even layer on paper towels. Sprinkle evenly with kosher salt; let rest 15 minutes.
Meanwhile, preheat oven to 400 degrees F and grease a 3.5-quart baking dish.
Add all of the "Herb Cream" ingredients EXCEPT sour cream and cheese to a medium saucepan. Bring to a gentle simmer for 3 minutes, stirring often; it will be very thick. Reduce heat to low and add ½ cup Gruyere, stir until melted. Remove from heat and whisk in sour cream until smooth; set aside.
Wipe the salt off of the zucchini slices with a paper towel and dab off the water, pressing firmly.
Arrange half of the zucchini slices in the prepared baking dish. Evenly top with half of the herb cream (it will be thick but will thin once baked). Top with remaining zucchini and remaining cream. Wiggle the dish so that the cream soaks in between the slices.
Top evenly with 1/4 cup Gruyere and shredded mozzarella. Cover the baking dish with foil and bake at 400 degrees for 30 minutes.
Meanwhile, melt 1 tablespoon butter in 1 tablespoon olive oil over medium heat in a skillet. Add panko and stir to coat. Continue cooking until crumbs turn golden brown. Transfer to a plate; set aside.
After 30 minutes, remove baking dish from the oven. Remove the foil, top with panko and bake uncovered for an additional 5 minutes or until cheese is melted and bubbling and the zucchini is tender crisp. Remove from oven and let stand 5 minutes before serving. Season with salt and pepper as needed (taste first!).