Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper for easy cleanup.
Slice the chicken breast and beef sirloin into uniform 1-inch strips to ensure they cook at the same rate.
In a large mixing bowl, combine the sliced chicken, beef, broccoli florets, and sliced bell peppers.
Drizzle the olive oil over the mixture and add the minced garlic and finely chopped fresh rosemary.
Season everything with the sea salt and black pepper, then toss well until all ingredients are evenly coated.
Spread the mixture in a single layer on the prepared baking sheet, ensuring the pieces are not overlapping so they roast rather than steam.
Roast for 15 to 18 minutes, or until the chicken is cooked through and the vegetables are tender-crisp with slightly charred edges.
Remove from the oven and allow the meat to rest for 2 minutes before serving to ensure every bite remains juicy.