Preheat the oven to Broil, drizzle olive oil in a skillet, and place the skillet in the oven to warm.
Mix the salt, pepper, and garlic powder in a small bowl. Line a baking sheet with parchment paper and lightly spray with nonstick spray. Set aside.
Trim the ends from the asparagus and spread them on the baking sheet. Drizzle with olive oil and sprinkle lightly with seasoning mixture. Set aside.
Season both sides of the steaks with the remaining spice mixture and set aside.
Remove the cast-iron skillet and carefully place the steaks inside. Use caution, as the skillet will sizzle and splatter. Scatter the chopped mushrooms over the steaks.
Return the skillet and asparagus pan to the oven and broil for 5 minutes. Flip the steaks, add thyme sprigs, and broil for another 5 minutes.
Check doneness and continue cooking in 2-minute intervals if needed, flipping each time, until the desired doneness is reached.
Remove asparagus when the ends begin to crisp, and the stalks are tender. Transfer the steaks to a cutting board and let them rest for about 10 minutes.
Top the steaks with small pads of butter, spoon the mushrooms over the top, garnish with fresh parsley, and remove the thyme sprigs if desired before serving.