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The night before cooking and serving, add the marinade ingredients to a large sealable plastic food storage bag. Seal and shake to combine. Add the meat, mushrooms, artichokes, olives, roasted red pepper, onions, and tomatoes to the bag. Seal and shake to combine. Let the bag sit in the refrigerator for at least an hour and up to a day before cooking.
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Heat a large skillet over high heat for a few minutes.
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Add the contents of the bag to a large, deep baking dish. Remove the beef and add it to the hot pan. Cook for a minute or two on each side and then remove from the pan. Place on a plate off to the side to rest while you cook the vegetables.
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Add the vegetables along with the marinade to the hot pan. Cook for ten to fifteen minutes, stirring occasionally.
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Add the spinach and cook another two to three minutes until just wilted. Stir to incorporate.
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Add the beef back to the pan, shut the heat off, and give the dish one final stir to incorporate everything. Serve the fajita mixture in pita pocket halves with a bit of crumbled feta or goat cheese on top.