In a large bowl or plastic bag, whisk together the olive oil, minced garlic cloves, dijon mustard, and rosemary. Add the steak and stir to coat evenly in the marinade. Season with kosher salt and black pepper. Cover and place in the fridge for at least 30 minutes and up to 24 hours.
Meanwhile, chop the bell peppers, shallots, and zucchini into even pieces. You want them to be similar in size to the steak so they bake evenly. For the mushrooms, wipe them clean instead of running them under water. This will reduce the moisture content.
Preheat oven to 400 degrees Fahrenheit. Thread vegetables and beef on metal or wood skewers, alternating as you go. Place on a baking sheet, covered with aluminum foil for easy clean up. Bake at 400℉ for 10-15 minutes. For medium, check with a meat thermometer, which should fall between 130℉-145℉. I recommend pulling them out on the lower end of 130℉ because it will raise if you broil them.
Turn oven to Broil on High heat and broil for 1-2 minutes, until charred in spots, flipping just once to get both sides. Watch carefully so the kabobs don't burn. Serve warm!
Notes
Use metal skewers, or if using wood, soak in water for 30 minutes.
For migraine diet friendly, use a dijon without wine/sulfites. If not following a migraine diet, red onion can be substituted for shallots. I really love the lighter flavor of shallots though!
Vegetables can be substituted! If you hate mushrooms, just sub those out with more zucchini, summer squash, cherry tomatoes, etc.
For medium, cook on the lower end of the cooking time, or until the internal temp is around 130 degrees Fahrenheit. Remember broiling will cook the meat more too!