Preheat your oven to 450°F and line a large baking sheet with parchment paper or foil for easy cleanup. While the oven heats, trim the asparagus by snapping off the woody ends (they'll naturally break at the right spot), then cut into 2-inch pieces for even cooking. Slice the mushrooms into even pieces about 1/4-inch thick—consistent sizing ensures they roast at the same rate. Strip the thyme leaves from the stems and roughly chop them, setting aside.

Place the cut asparagus and sliced mushrooms on the prepared baking sheet. Drizzle with the olive oil and sprinkle with salt and black pepper, then toss everything together with your hands or two spoons until the vegetables are evenly coated. Spread them into a single layer, giving them room to roast rather than steam—don't overcrowd the pan.

Roast the vegetables uncovered for 15 minutes. During this time, the mushrooms will release their moisture and then it will evaporate, allowing them to brown beautifully. The asparagus tips will begin to caramelize. I like to give the pan a gentle shake halfway through to ensure even cooking, though you can leave it undisturbed if you prefer.

Remove the pan from the oven and dot the roasted vegetables with butter pieces, then scatter the chopped thyme over the top. Return to the oven and roast for another 5 minutes until the butter melts and becomes fragrant, coating the vegetables with richness and herb flavor.

Remove the pan from the oven and immediately drizzle the balsamic vinegar over the hot vegetables—it will reduce slightly from the residual heat, intensifying its flavor. Finish with a pinch of flaky sea salt to add brightness and textural contrast. Serve warm directly from the pan or transfer to a serving dish.