Add the garlic to the skillet. Once it sizzles, add the onion and bell peppers and cook, stirring occasionally, until they soften slightly, about 4 minutes. Add the pepperoncini, tomatoes, oregano, red pepper flakes, and 3/4 c. water and stir to combine. Bring to a rapid simmer, then nestle the steak in the sauce and simmer, turning once, until medium rare, about 7 minutes. Transfer the steak to a cutting board and let rest about 5 minutes. Continue simmering the sauce until thickened, about 3 more minutes.
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