Slice zucchini into half circles. Dice onion. Cut gruyere cheese into cubes.
Melt 1 Tbsp. butter in a large nonstick pan. Saute the zucchini and onions until tender. Add the salt and combine well. Transfer the zucchini mixture to a colander over the sink. Allow to drain for 20 minutes.
In a large bowl, combine the sour cream, cream cheese and Parmesan. Gently fold the zucchini mixture into the sour cream mixture. Transfer to an 8x8 baking dish. Arrange gruyere cubes over top.
Melt the remaining 2 Tbsp. of butter in a small bowl. Combine with the panko bread crumbs. Sprinkle panko mixture evenly over the zucchini. Bake 40 minutes.