In a large nonstick skillet over medium heat, add olive oil. When hot, add diced onion and cook until it begins to turn brown, about 3 to 4 minutes.

Add zucchini, butter, salt, pepper, and red pepper flakes to pan, tossing to coat and spreading into a single layer. Cook for 5 to 7 minutes, flipping once halfway through, or until zucchini is tender and lightly browned around the edges.

Add minced garlic and stir to incorporate. Cook for 30 seconds, or until garlic is fragrant. Remove from heat and serve immediately.