Preheat oven to 425℉. Lightly grease a sheet pan (or line with parchment paper) and set aside.

Allow zucchini to lay flat naturally on the countertop, then slice off about ¼ inch at each end to remove stems. Place a chopstick (or thin wooden spoon) on either side. Slice zucchini across the top into ¼" sections, slicing until you hit the chopsticks, leaving the bottom of the zucchini intact.

Place sliced zucchini onto prepared sheet pan.

In a small bowl, add olive oil, red wine vinegar, Dijon mustard, oregano, salt, and red pepper flakes. Whisk to combine, then brush liberally over each zucchini.

Bake for 15 minutes.

Remove from oven and add gruyere cheese into every other space of the zucchini. Top with parmesan cheese.

Bake for another 10 minutes, or until cheese is melted and zucchini is cooked to your liking (add more time if you'd like softer zucchini).