Roasted Asparagus and Mushrooms

INGREDIENTS

pound asparagus, trimmed and halved crosswise 

1 lb

hen-of-the-woods or oyster mushrooms, or 8 ounces morel mushrooms 

2 tbsp

extra-virgin olive oil 

1 tsp

kosher salt 

2 tbsp

unsalted butter 

1 tbsp

fresh thyme leaves 

1 cup

loosely packed watercress or arugula (about 1/4 ounce) 

Flaky sea salt (preferably Maldon) 

Aged balsamic vinegar (preferably San Giacomo Essenza Balsamic), for drizzling 