Heat 2 tablespoons olive oil in a large skillet over medium-high heat. Add the thawed reserved meat-loaf mixture (4 cups) and cook, breaking up with a spoon, until browned, 5 to 7 minutes. Add 1 seeded, cored, and chopped green pepper (1/4-inch pieces), 1/2 cuptomato paste, 1 tablespoon brown sugar, and 1/2 teaspoon kosher salt. Cook, stirring occasionally, about 5 minutes. Remove from heat and stir in chopped tomato, red pepper flakes, and parsley. Serve on toasted buns with lettuce and sliced tomato.