Preheat the oven to 425° and lightly grease a large baking sheet if needed. (If you have an older baking sheet, you can cover it with parchment paper to prevent sticking.)
Snip the stems off green bean pods, if any, and add them into a large mixing bowl.
Quickly rinse mushrooms under cold running water and pat them dry right away. Cut larger mushrooms in half and add them to the mixing bowl as well.
Add oil and seasoning to the bowl and toss green beans and mushrooms to coat them evenly.
Spread the vegetables in the prepared baking sheet and try not to overcrowd them. If needed, use two medium baking sheets.
Roast the vegetables for about 18-20 minutes. Roast them a few minutes more if you like them softer or a couple minutes less if you like them crunchier.