Preheat your oven to 400°F (200°C) and line a large sheet pan with parchment paper.
Peel the carrots and parsnips, then slice them into 1/2-inch thick rounds.
Trim any excess fat from the beef sirloin and cut it into uniform 1-inch cubes.
In a large mixing bowl, combine the beef, carrots, and parsnips.
Drizzle with olive oil and add the minced garlic, dried rosemary, sea salt, and black pepper.
Toss everything thoroughly until the beef and vegetables are evenly coated in the oil and seasonings.
Spread the mixture in a single layer on the prepared sheet pan, ensuring pieces are not overcrowded.
Roast for 18 to 22 minutes, tossing halfway through, until the vegetables are tender and the beef reaches your desired level of doneness.
Remove from the oven and let the beef rest for 2 minutes before serving.