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Preheat oven to 350 levels F.
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Pulse oats in a food processor to invent a inaccurate meal. Switch to an unlimited bowl and web converse aside. Pulse mushrooms within the food processor till minced, 10 to 15 pulses.
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Heat 1 Tbsp. oil in broad nonstick skillet over medium-excessive warmth. Add onion, bell pepper, and garlic; cook dinner, stirring every so generally, till subtle, 3 to 4 minutes. Add the mushrooms; cook dinner, stirring every so generally, till browned, 3 to 5 minutes. Add vinegar; cook dinner, stirring and scraping up any browned bits, till the liquid has evaporated, 1 to 2 minutes. Let chilly for 10 minutes.
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Add the mushroom mixture to the bowl with the oats, alongside with beef, egg, mustard, Worcestershire, salt, and pepper. Gently mix till blended; invent no longer overmix.
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Brush an 8×4-inch loaf pan with the final 1 tsp. oil; switch the beef mixture to the pan and restful right into a loaf. Mix ketchup and cayenne in a little bowl; brush over the pinnacle of the loaf.
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Bake till an on the spot-be taught thermometer inserted within the thickest half registers 160 levels F, 50 to 60 minutes. Let stand for at least 10 minutes before taking away from the pan. Using a serrated knife, fastidiously minimize the loaf into 6 slices.
