Step 1

Preheat the oven to 400 degrees F (200 degrees C). Line a baking sheet with foil and spray with cooking spray.

Step 2

Gently clean off mushrooms with a damp paper towel. Cut off stems and discard. Cut any larger mushrooms in half so mushrooms are all about the same size and place in a medium bowl.

Step 3

Cut the white parts of the green onions into 1/2-inch pieces and add to the mushrooms. Thinly slice the green parts and set aside for later.

Step 4

Whisk oil, vinegar, thyme, 1/2 teaspoon salt, and pepper together in a small bowl. Drizzle over the mushroom mixture and toss to coat. Place in a single layer on the prepared baking sheet.

Step 5

Roast in the preheated oven for 10 minutes. Flip mushrooms over and roast for an additional 10 minutes. Remove from the oven and season with more salt, if desired. Sprinkle with reserved green onion parts.