Preheat your oven to 350°F (175°C). Lightly grease a 9x13-inch baking dish with nonstick cooking spray and set it aside.
In a large skillet over medium-high heat, cook the ground beef and finely chopped onion until the meat is browned and the onion is tender, which should take about 8 minutes. Drain off any excess grease from the skillet.
Stir in the taco seasoning mix and 1 cup of water, then bring the mixture to a simmer. Cook for 2-3 minutes, stirring occasionally, until the sauce has thickened. Remove the skillet from the heat.
Stir the drained black beans, drained corn, and the entire undrained can of diced tomatoes with green chilies into the beef mixture until everything is thoroughly combined.
Arrange half of the corn tortilla strips in a single layer on the bottom of the prepared baking dish. Top with half of the beef mixture, spreading it into an even layer.
Spread the 1 cup of sour cream evenly over the beef layer. Top with the remaining beef mixture and then the remaining tortilla strips. Sprinkle the 1 cup of shredded cheese evenly over the top.
Bake uncovered for 25-30 minutes, or until the casserole is bubbly around the edges and the cheese is completely melted and lightly browned.
