Cheesy Beef Enchilada Casserole

4 tablespoons butter (1/2 stick)
¼ cup all-purpose flour
3 cups reduced-fat milk
2 ½ cups shredded sharp Cheddar cheese
⅓ cup grated Parmesan cheese
½ teaspoon salt
2 teaspoons avocado oil or canola oil
8 ounces 90%-lean ground beef
1 large onion, diced
1 medium red bell pepper, diced
1 cup frozen corn kernels
1 tablespoon ground cumin
2 teaspoons chili powder
1 teaspoon garlic powder
1 (4 ounce) can diced green chiles
8 each corn tortillas, cut into quarters
1 large tomato, seeded and chopped
¾ cup red-wine vinegar
5 tablespoons water
1 ½ tablespoons sugar
1 tablespoon Dijon mustard
1 large clove garlic
2 teaspoons dried basil
2 teaspoons dried oregano
½ teaspoon salt
½ teaspoon ground pepper
1 ¾ cups extra-virgin olive oil
1 (14 ounce) package coleslaw mix
½ cup chopped fresh cilantro, divided

Directions

1. To prepare cheese sauce: Melt butter in a medium saucepan over medium heat. Add flour and whisk constantly until thickened and just starting to turn light brown, 2 to 3 minutes. Slowly add milk, whisking constantly. Cook, stirring often, until the mixture is thick enough to coat the back of a spoon, 10 to 12 minutes. (Do not boil.) Remove from heat and, a handful at a time, whisk in Cheddar and Parmesan until smooth. Stir in 1/2 teaspoon salt. (Reserve 2 1/2 cups for another use.)
2. Preheat oven to 400 degrees F. Coat a 9-inch (or similar size) baking dish with cooking spray.
3. To prepare enchiladas: Heat avocado (or canola) oil in a large skillet over medium-high heat. Add beef, onion and bell pepper and cook, crumbling the beef with a wooden spoon, until it is no longer pink and the vegetables are tender, 5 to 8 minutes. Stir in corn, cumin, chili powder and garlic powder; cook, stirring occasionally, until the corn is heated through and the spices are fragrant, about 1 minute. Remove from heat.
4. Stir green chiles into 1 1/2 cups of the warm cheese sauce. Arrange one-third of the tortilla wedges in the prepared baking dish, overlapping as necessary to fit. Top with half the beef mixture and 1/2 cup of the cheese sauce. Repeat with half the remaining tortillas, the remaining beef mixture and 1/2 cup of the cheese sauce. Top with the remaining tortillas, the remaining 1/2 cup cheese sauce and tomato. Bake until bubbling, about 15 minutes.
5. Meanwhile, prepare dressing: Combine vinegar, water, sugar, mustard, garlic, basil, oregano, salt and pepper in a blender. Puree until smooth. With the motor running, slowly add oil and puree until creamy. (Measure out 2/3 cup and transfer the remaining dressing to a large mason jar; refrigerate for up to 1 week.)
6. To prepare slaw & serve: Toss coleslaw mix with 1/4 cup cilantro and 2/3 cup dressing in a medium bowl. Top the casserole with the remaining 1/4 cup cilantro and cut into 4 servings. Serve with the slaw.