In a large bowl, season the sirloin liberally with salt and pepper. Toss to coat.
Heat a bit of oil in a large pot over medium-high heat until shimmering. Place half of the cubed meat in the pan, spacing out to avoid overcrowding. Sear on each side for 1 minute, until well-browned. Transfer the seared beef to a bowl and repeat with the remaining meat.
Reduce the heat to medium. Add the onion, mushrooms, and a pinch of salt. Sauté until all of moisture has been release from the mushrooms and evaporated and the onions have caramelized, about 12 minutes.
Add the beef broth and scrape up any browned bits from the bottom of the pan.
Once the liquid has thickened slightly, mix a few tablespoons of the hot broth with the sour cream in a small bowl--this will temper the sour cream and keep it from curdling. Add the sour cream mixture to the broth and mix until incorporated.
Once the liquid has thickened slightly, add the seasoned sirloin and any juices that have collected in the bowl.
Stir the the contents of the pot until the meat has cooked through, about 5 minutes.
Use a vegetable peeler to slice the zucchini into ribbons.
Heat a drizzle of oil in a medium pan over medium-high heat.
Add the zucchini ribbons and a pinch of salt and toss until the zucchini has wilted slightly, about 1 minute. Immediately transfer the zucchini to serving dishes to stop cooking.
Top the zucchini with the stroganoff and a sprinkle of chives.