Beef Tostadas with Quick-Pickled Onions

Thanks to the quick-pickled onions, which are so simple that you'll cook them over and over again, this Texas-Mexican steak takes on a vibrant hue. To add spice, top with the jalapeno slices that were used in the pickling brine.
Combine the onion, vinegar, 1/4 cup water, sugar, 1/4 teaspoon salt, and jalapeno slices in a saucepan over high heat; bring to a boil. Cook for 1 minute; remove from heat and allow to simmer until ready to serve.While the onion mixture is cooking, heat a large skillet over medium-high heat. Brush the mold with cooking spray. Add the ground beef, ground chili pepper, cumin, garlic, paprika and remaining 1/4 teaspoon salt to the pan. Cook for 5 minutes or until they are lightly browned, stirring occasionally until they crumble. Drizzle with oil.Drain the onion mixture. Sprinkle the beef mixture, onion mixture, sour cream and coriander evenly over the tostadas.