Preheat your oven to 375°F (190°C). Toss the cubed butternut squash with olive oil and spread it out on a baking sheet. Roast for about 25 minutes or until tender. I've learned that roasting the squash beforehand enhances its natural sweetness and adds a delightful caramelization that's key to the casserole's depth of flavor.

While the squash is roasting, heat a large skillet over medium heat. Add a splash of olive oil, followed by the diced onions. It's tempting to rush, but slow-cooking the onions until they're caramelized unlocks a depth of flavor that's truly worth the wait. Stir in the minced garlic, cumin, chili powder, and smoked paprika, giving them a quick toast in the pan to awaken their aromatic oils.

Add the black beans, corn, and diced tomatoes to the skillet. Gently toss and let them simmer for about 10 minutes. This allows the flavors to meld together and the tomatoes to release their natural juices, developing a luscious base for the casserole.

In a large bowl, combine the roasted butternut squash, the bean and tomato mixture, half of the shredded cheese, cilantro, and green onions. Pour in the chicken broth to moisten the mixture — this little secret I learned ensures that the casserole doesn't dry out while baking.

Layer the bottom of a greased 9x13 inch baking dish with half of the corn tortilla strips. Press down gently after each addition. This helps in achieving those beautiful, defined layers when you slice into the dish. Spoon half of the squash mixture over the tortillas, then repeat the layers, finishing with the remaining shredded cheese on top.

Bake in the preheated oven for 35-40 minutes, or until the cheese is bubbly and golden brown. I remember overcooking it the first time around, thinking it needed more time. It's essential to trust the process and the specified timings to avoid a dry casserole.

Let the casserole rest for about 10 minutes before slicing. This relaxation step allows the layers to set, making it easier to serve. Garnish with sliced avocado and a dollop of sour cream if desired, and enjoy the fiesta of flavors in your mouth!