Nutrition per serving may change if servings are adjusted.
½ cup quick-cooking oats
4 ounces cremini mushrooms (1½ cups)
1 tablespoon olive oil plus 1 tsp., divided
½ cup finely chopped onion
½ cup finely chopped green bell pepper
6 cloves garlic, minced
2 tablespoons cider vinegar
1½ pounds lean ground beef
1 large egg, lightly beaten
1 tablespoon Dijon mustard
1 tablespoon Worcestershire sauce
¼ teaspoon salt
½ teaspoon ground pepper
¼ cup ketchup
¼ teaspoon ground cayenne pepper
Preheat oven to 350°F.
Pulse oats in a food processor to form a coarse meal. Transfer to a large bowl and set aside. Pulse mushrooms in the food processor until minced, 10 to 15 pulses.
Heat 1 Tbsp. oil in large nonstick skillet over medium-high heat. Add onion, bell pepper, and garlic; cook, stirring occasionally, until tender, 3 to 4 minutes. Add the mushrooms; cook, stirring occasionally, until browned, 3 to 5 minutes. Add vinegar; cook, stirring and scraping up any browned bits, until the liquid has evaporated, 1 to 2 minutes. Let cool for 10 minutes.
Add the mushroom mixture to the bowl with the oats, along with beef, egg, mustard, Worcestershire, salt, and pepper. Gently mix until combined; do not overmix.
Brush an 8x4-inch loaf pan with the remaining 1 tsp. oil; transfer the beef mixture to the pan and smooth into a loaf. Combine ketchup and cayenne in a small bowl; brush over the top of the loaf.
Bake until an instant-read thermometer inserted in the thickest part registers 160°F, 50 to 60 minutes. Let stand for at least 10 minutes before removing from the pan. Using a serrated knife, carefully cut the loaf into 6 slices.