How to Make It
Preheat oven to 450°F. Line a rimmed baking sheet with heavy-duty aluminum foil. Arrange asparagus and mushrooms in an even layer on prepared baking sheet. Drizzle with olive oil, sprinkle with kosher salt, and toss to coat. Roast in preheated oven, stirring occasionally, until charred and tender, about 15 minutes. Add butter and thyme leaves. Once butter is melted, toss vegetables to coat, and roast until glazed, about 5 minutes.
Arrange roasted vegetables on a platter, top with watercress, and season with flaky sea salt to taste. Drizzle with balsamic vinegar.