In a large mixing bowl, whisk together olive oil, rosemary, and minced garlic.

Add balsamic vinegar, lemon juice, Dijon mustard, salt, black pepper, and optional crushed red pepper flakes. Mix well to form the marinade.

Add the cubed sirloin steak to the marinade, tossing to coat. Cover and refrigerate for at least 1 hour or up to 4 hours.

While marinating, chop the bell peppers and rinse cherry tomatoes. Pat them dry with a kitchen towel.

Thread steak cubes, bell peppers, and cherry tomatoes onto skewers, alternating them for a colorful arrangement.

Preheat the grill to medium-high heat (375°F to 425°F) and lightly brush grates with oil.

Cook the kebabs on the grill for 8–10 minutes, turning occasionally until the steak has a char and veggies are slightly softened.

Optional: Brush additional marinade onto kebabs while grilling for added flavor.

Remove kebabs from the grill when done, let them rest for 5 minutes before serving.

Garnish with fresh rosemary or parsley and serve alongside your favorite sides.