Bring broth to a simmer in a medium saucepan. Whisk milk and flour in a small bowl until smooth. Add to the broth and cook over medium heat, whisking constantly, until thickened and smooth, about 3 minutes. Remove from the heat and stir in yogurt, tomatoes, chiles, cilantro (or parsley), chili powder, oregano and cumin. Season with salt and pepper.
Heat oil in a large nonstick skillet over medium-high heat. Add onion, bell pepper and garlic; cook, stirring occasionally, until tender-crisp, about 3 minutes.
Line the bottom of a shallow 3-quart baking dish with half the tortillas. Top with half the chicken and half the onion mixture. Spoon half of the sauce evenly over the top. Repeat layers with remaining tortillas, chicken, onion mixture and sauce. Sprinkle with Cheddar. Bake until bubbly, 25 to 30 minutes.